you're probably using some 3rd party software for clipboard control?
I'm using Firefox on Windows 10 x64, without clipboard apps.
I've been having this bug for years on two different computers.
Also, sometimes you want to copy the video URL in order to share it or do something else with it, so the SVPtube popup would be disruptive in those situations.
A right click menu option like 'Launch manually' would solve everything.