(In reply to Frederik Braun [:freddyb] from comment #2)
I have observed a similar problem, when trying to start pip on a video that has not yet loaded.
Yep, thanks for the tip. I can easily reproduce this on youtube by limiting the internet connection, select a random video and activate PiP while the circle is spinning (the video is loading).
That being said I tried searching for a regression range on Windows 10x64, but I didn't find one because the issue is happening way back to Firefox 67.0a1 (20190212195904), which is the last build that has "media.videocontrols.picture-in-picture.enabled" pref for picture in picture. So I think it's safe to assume that the issue is not a regression. Removing the flags.