I can't watch videos on- or off-line in the Dropbox app for Android. I think this bug/series of bugs started when I updated to Android 12 in October/November of last year. I've just been hoping the bug will be fixed in the next version of the app, but it never was fixed. I'm hoping someone else has experienced the bugs. It's hard to explain exactly what is happening but I will try here.
A giant thumbnail will obscure the currently playing video, taking up about half the screen, depending on the resolutions of the currently playing video and the thumbnailed video. This thumbnail seems to always come from the same folder the video is in. I think it only happens when the video/screen is in landscape.
When videos do start playing correctly, the UI (seek bar, play/pause, favorite, share, etc) won't go away automatically. When tapping somewhere in the video to hide it, it sometimes will just toggle between two different UI states, instead of hiding it completely. This seems to be random and the only way to fix it has been to clear the app from recents (which I think closes it) and restart it.
The seek bar in videos is also always in the middle of my screen instead of at the bottom.