@YouTube app won't let you go back to last video using back button anymore. You literally can't go back now. So if you wrongly tap on another video by mistake you're screwed. Where are the UX team and how the heck did they let that change go to production?
We recently made an update to the back button–now it takes you back to the watch page. A second back button tap will minimize the video player and a final tap will exit the app completely. You can share feedback about the change directly from YouTube: https://goo.gl/zcddRh