I created a new profile "Test" as a copy of "Automatic" and was able to change the "nvof_" values in that profile. I selected the "Test" profile during playback.
I tried different values of "nvof_grid" such as 12, 16, 24, 48, and 64, but there was no difference.
I tried changing "nvof" from "-1" to "1" in the profile but it had no effect.
Changing "mv_grid" also had no effect with Optical Flow enabled.
However I noticed something strange while testing. With Optical Flow enabled on default settings, it randomly works for a few seconds while the scene complexity is low. During a panning shot, the video becomes smooth with interpolation while the background is simple, and when the background becomes complex, interpolation stops.