Playing videos immediately jumps to end and loses saved spot

Win10, Firefox 73.0.1, adblock disabled for GB.

When playing a video, it will often buffer for a second, and then jump to a black screen with the replay button available in the lower left. Clicking replay starts the video from the beginning, regardless of whether or not the video had a spot saved from before. This happens both with videos that I've never watched and with videos I've closed the browser in the middle of.

I'm having the same issue, but randomly halfway through videos. :/