Google automatically syncs your Play Movies and TV content with your YouTube account. This allows you to watch all of your Play Movies & TV content on YouTube without having to redeem your content. However, if you choose to watch said content on YouTube, you may be limited to 480p playback. You will not be able to play HD movies even if your original purchase was in HD quality.
The temporary cap appears to have been in place since the start of last week and YouTube claims the lack of HD playback is the result of a “technical glitch.” However, YouTube did not mention when HD playback will be restored. It should be noted that this issue affects both rented and purchased content.
The cap only occurs when viewing directly on the YouTube website and does not affect users using the app through Android, iOS, Android TV, or Chromecast. So if you really want to read in HD, you will have options.
via Android Police