... and the app is actually written by Google, so they may be arguing as to whether this is a YouTube app bug or an X1 platform bug.
OR, another possibility is that this could be intentional behaviour on Google's part, and the back button was disabled to prevent you from interrupting ads.
This should be an easy, simple regression to fix... and I am REALLY curious as to WHY such an annoying bug has not been fixed yet.
This appears to be corrected now. Anyone else want to give it a test?