I’ve got a fix here for the video restarting when scrolling to different posts:
https://github.com/discourse/discourse/commit/65d7103bdc20d597decaeefa6ae9c782d6d51aaa
As for it stopping when offscreen: that’s the result of our cloaking, which tries to save memory by unloading things as they scroll off. We might be able to increase the threshold before this happens, or perhaps add a check to see if there’s a video playing, but I think it’s less serious of a bug.