Hi, you can try to long press the YouTube icon for 1 second then select App info > Storage > Clear Catch, if that option doesn't help solving your issue the repeat the same steps but press Clear Catch ( The second solution will delete your downloaded videos and will sign you out )
True bro i also faced the same issue when you are trying screen on screen mode of youtube. But with recent youtube update, it's solved. Hope yours solved as well.....
