Да, что-то неправильно отрабатывает. Надо разбираться.
Проверил на такой же версии macOS: перекодирование работает корректно.
В случае успешного завершения перекодирования журнал должен быть такой:
19:11:47.160: ===== Starting mpv ======
19:11:47.160: Command line: /usr/local/bin/mpv /Volumes/Data/Загрузки/Флешмоб + стих.mp4 -o /Volumes/Data/Загрузки/Флешмоб + стих..SVP.temporary.mkv --no-audio --no-sub --no-sub-auto --input-ipc-server=/tmp/mpvencodesocket --input-media-keys=no --vf=vapoursynth:/Users/andrey/Library/Application Support/SVP4/scripts/ffff.py:4:2 --of=matroska --ovc=libx264 --ovcopts=crf=18,preset=slow,threads=2
19:11:47.336: Playing: /Volumes/Data/Загрузки/Флешмоб + стих.mp4
19:11:47.359: (+) Video --vid=1 (*) (h264 1920x1080 29.970fps)
19:11:47.359: Audio --aid=1 --alang=eng (*) (aac 2ch 48000Hz)
19:11:47.816: VO: [lavc] 1920x1080 yuv420p
19:11:47.817: [vo/lavc] Opening encoder: libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 [libx264]
19:11:47.819: [ffmpeg] libx264: MB rate (195840000) > level limit (16711680)
19:11:47.839: [encode] Opening output file: /Volumes/Data/Загрузки/Флешмоб + стих..SVP.temporary.mkv
19:11:47.840: [encode] Opening muxer: Matroska [matroska]
19:11:47.893: V: 00:00:00 / 00:04:34 (0%) {8.8min 7.4MB}
19:12:48.038: V: 00:00:11 / 00:04:34 (4%) {23.5min 10.2fps 67.0MB}
19:13:48.079: V: 00:00:18 / 00:04:34 (6%) {27.2min 8.9fps 212.6MB}
19:14:48.270: V: 00:00:26 / 00:04:34 (9%) {28.3min 8.4fps 263.5MB}
19:15:48.618: V: 00:00:32 / 00:04:34 (11%) {30.1min 7.8fps 321.5MB}
19:16:48.680: V: 00:00:39 / 00:04:34 (14%) {30.0min 7.6fps 336.2MB}
19:17:48.962: V: 00:00:46 / 00:04:34 (17%) {29.1min 7.6fps 345.0MB}
19:18:49.006: V: 00:00:54 / 00:04:34 (19%) {28.1min 7.7fps 334.3MB}
19:19:49.077: V: 00:01:01 / 00:04:34 (22%) {27.6min 7.6fps 339.0MB}
19:20:49.100: V: 00:01:07 / 00:04:34 (24%) {27.6min 7.4fps 368.3MB}
19:21:49.254: V: 00:01:14 / 00:04:34 (27%) {26.9min 7.3fps 370.4MB}
19:22:49.268: V: 00:01:21 / 00:04:34 (29%) {26.1min 7.3fps 375.7MB}
19:23:49.379: V: 00:01:29 / 00:04:34 (32%) {24.9min 7.3fps 369.8MB}
19:24:49.411: V: 00:01:37 / 00:04:34 (35%) {23.4min 7.4fps 360.6MB}
19:25:49.672: V: 00:01:46 / 00:04:34 (38%) {22.2min 7.5fps 350.4MB}
19:26:49.710: V: 00:01:55 / 00:04:34 (41%) {20.8min 7.6fps 348.5MB}
19:27:49.803: V: 00:02:02 / 00:04:34 (44%) {19.9min 7.6fps 346.9MB}
19:28:49.828: V: 00:02:09 / 00:04:34 (47%) {18.9min 7.6fps 341.1MB}
19:29:49.902: V: 00:02:16 / 00:04:34 (49%) {18.1min 7.5fps 342.8MB}
19:30:49.907: V: 00:02:22 / 00:04:34 (51%) {17.7min 7.4fps 361.9MB}
19:31:49.917: V: 00:02:28 / 00:04:34 (54%) {16.9min 7.4fps 369.0MB}
19:32:49.949: V: 00:02:36 / 00:04:34 (57%) {15.7min 7.4fps 367.7MB}
19:33:50.131: V: 00:02:43 / 00:04:34 (59%) {14.8min 7.4fps 363.3MB}
19:34:50.169: V: 00:02:51 / 00:04:34 (62%) {13.7min 7.4fps 361.0MB}
19:35:50.182: V: 00:02:58 / 00:04:34 (65%) {12.8min 7.4fps 358.7MB}
19:36:50.250: V: 00:03:05 / 00:04:34 (67%) {12.0min 7.3fps 360.3MB}
19:37:50.279: V: 00:03:12 / 00:04:34 (70%) {11.1min 7.3fps 359.8MB}
19:38:50.307: V: 00:03:20 / 00:04:34 (73%) {9.9min 7.4fps 355.3MB}
19:39:50.336: V: 00:03:28 / 00:04:34 (75%) {8.9min 7.4fps 354.2MB}
19:40:50.417: V: 00:03:35 / 00:04:34 (78%) {7.8min 7.4fps 351.2MB}
19:41:50.417: V: 00:03:42 / 00:04:34 (81%) {6.9min 7.4fps 352.5MB}
19:42:50.488: V: 00:03:50 / 00:04:34 (84%) {5.8min 7.4fps 348.4MB}
19:43:50.667: V: 00:03:58 / 00:04:34 (86%) {4.8min 7.4fps 349.8MB}
19:44:50.847: V: 00:04:05 / 00:04:34 (89%) {3.9min 7.4fps 350.0MB}
19:45:51.215: V: 00:04:12 / 00:04:34 (92%) {2.9min 7.4fps 350.0MB}
19:46:51.321: V: 00:04:20 / 00:04:34 (94%) {1.9min 7.4fps 349.5MB}
19:47:51.446: V: 00:04:27 / 00:04:34 (97%) {0.8min 7.4fps 347.9MB}
19:48:44.585: [encode] video: encoded 369378904 bytes
19:48:44.585: [encode] audio: encoded 0 bytes
19:48:44.585: [encode] muxing overhead 129053 bytes
19:48:44.598: Exiting... (End of file)
19:48:44.620: ===== Starting ffmpeg ======
19:48:44.620: Command line: /usr/local/bin/ffmpeg -y -i /Volumes/Data/Загрузки/Флешмоб + стих.mp4 -i /Volumes/Data/Загрузки/Флешмоб + стих..SVP.temporary.mkv -map 1:v -map 0:a? -map 0:s? -vcodec copy -acodec copy -scodec copy /Volumes/Data/Загрузки/Флешмоб + стих..SVP.mp4
19:48:44.654: ffmpeg version 4.1 Copyright (c) 2000-2018 the FFmpeg developers
19:48:44.654: built with Apple LLVM version 10.0.0 (clang-1000.11.45.5)
19:48:44.654: configuration: --prefix=/usr/local/Cellar/ffmpeg/4.1_1 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gpl --enable-libmp3lame --enable-libopus --enable-libsnappy --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-opencl --enable-videotoolbox
19:48:44.654: libavutil 56. 22.100 / 56. 22.100
19:48:44.654: libavcodec 58. 35.100 / 58. 35.100
19:48:44.654: libavformat 58. 20.100 / 58. 20.100
19:48:44.654: libavdevice 58. 5.100 / 58. 5.100
19:48:44.654: libavfilter 7. 40.101 / 7. 40.101
19:48:44.654: libavresample 4. 0. 0 / 4. 0. 0
19:48:44.654: libswscale 5. 3.100 / 5. 3.100
19:48:44.654: libswresample 3. 3.100 / 3. 3.100
19:48:44.654: libpostproc 55. 3.100 / 55. 3.100
19:48:44.674: Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/Volumes/Data/Загрузки/Флешмоб + стих.mp4':
19:48:44.674: Metadata:
19:48:44.674: major_brand : avc1
19:48:44.674: minor_version : 0
19:48:44.674: compatible_brands: avc1mp42
19:48:44.674: creation_time : 2018-09-04T17:34:01.000000Z
19:48:44.674: Duration: 00:04:34.01, start: 0.000000, bitrate: 9438 kb/s
19:48:44.674: Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 9039 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
19:48:44.674: Metadata:
19:48:44.674: creation_time : 2018-09-04T17:34:01.000000Z
19:48:44.674: handler_name : Video Media Handler
19:48:44.674: encoder : AVC Coding
19:48:44.674: Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 384 kb/s (default)
19:48:44.674: Metadata:
19:48:44.674: creation_time : 2018-09-04T17:34:01.000000Z
19:48:44.674: handler_name : Sound Media Handler
19:48:44.679: Input #1, matroska,webm, from '/Volumes/Data/Загрузки/Флешмоб + стих..SVP.temporary.mkv':
19:48:44.679: Metadata:
19:48:44.679: COMPATIBLE_BRANDS: avc1mp42
19:48:44.679: creation_time : 2018-09-04T17:34:01.000000Z
19:48:44.679: MAJOR_BRAND : avc1
19:48:44.679: MINOR_VERSION : 0
19:48:44.679: ENCODER : Lavf58.20.100
19:48:44.679: Duration: 00:04:33.96, start: 0.000000, bitrate: 10790 kb/s
19:48:44.679: Stream #1:0
19:48:44.679: : Video: h264 (High), yuv420p(tv, bt709/unknown/unknown, progressive), 1920x1080 [SAR 1:1 DAR 16:9], 59.94 fps, 59.94 tbr, 1k tbn, 48k tbc (default)
19:48:44.679: Metadata:
19:48:44.679: DURATION : 00:04:33.960000000
19:48:44.680: Output #0, mp4, to '/Volumes/Data/Загрузки/Флешмоб + стих..SVP.mp4':
19:48:44.680: Metadata:
19:48:44.680: major_brand : avc1
19:48:44.680: minor_version : 0
19:48:44.680: compatible_brands: avc1mp42
19:48:44.680: encoder : Lavf58.20.100
19:48:44.680: Stream #0:0: Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709/unknown/unknown, progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 59.94 fps,
19:48:44.680: 59.94 tbr, 16k tbn, 1k tbc (default)
19:48:44.680: Metadata:
19:48:44.680: DURATION : 00:04:33.960000000
19:48:44.680: Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 384 kb/s (default)
19:48:44.680: Metadata:
19:48:44.680: creation_time : 2018-09-04T17:34:01.000000Z
19:48:44.681: handler_name : Sound Media Handler
19:48:44.681: Stream mapping:
19:48:44.681: Stream #1:0 -> #0:0 (copy)
19:48:44.681: Stream #0:1 -> #0:1 (copy)
19:48:44.681: Press [q] to stop, [?] for help
19:48:44.681: [mp4 @ 0x7fa5c68b4a00] Non-monotonous DTS in output stream 0:0; previous: 0, current: 0; changing to 1. This may result in incorrect timestamps in the output file.
19:48:45.181: frame= 9692 fps=0.0 q=-1.0 size= 230400kB time=00:02:41.66 bitrate=11675.1kbits/s speed= 323x
19:48:45.513: frame=16422 fps=0.0 q=-1.0 Lsize= 374076kB time=00:04:34.00 bitrate=11183.8kbits/s speed= 329x
19:48:45.513: video:360722kB audio:12845kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.136391%