Agreed. This is exceptionally annoying! Who thought this was a good idea? Why don't people copy proven video interface behavior from Google. Why go out of your way to annoy your users?

I get the annoyance, but especially on mobile, it conversely helps if you want to advance the video by as few frames as possible to catch a freeze-frame gag or something like that. If the UI immediately disappeared upon resume, you'd have to triple tap to immediately pause the video again. (On desktop you can just mash the keyboard or even use a dedicated "advance one frame" key, but on mobile that's not available.)