Whenever using the youtube app on my 65U6GR, I can try to rewind the video. Rewinding more than 20 seconds will trigger this bug most of the time.
What happens is you try to rewind and the video will play from the position you reminded to. However the loading icon will stay in the center of the screen, also the progress bar at the bottom will stay grey. After about 10 seconds the video will jump forwards to where the video was at before you tried to rewind it.
This makes rewinding very frustrating as sometimes you have to repeat this process 2 to 3 times before the video will stay at the selected playback position. Try it with a video that is half watched then rewind. If the part of the video you are rewinding to isn't cached it most likely won't load right.
This seems more of an app issue rather than a tv or internet issue. Almost like the youtube app can't verify the playback position correctly and just gives up and skips forward again.
So far I have reinstalled the app, switched from my 2.4g to 5g internet at 200mpbs. Tried pausing the video before rewinding. Restarting the tv does help the first time or so rewinding while using the app then the app fails to rewind again.
I don't want to factory reset my tv because, like I said, it seems like an app issue more than the tv itself.