MRA58U is an update that fixes the video playback issue with the MRA58K build on the 2013 Nexus 7. Video would hang after a few seconds, with audio continuing to play on almost any app except Google apps. Netflix and Vudu were affected, among others.
Google appeared to stop the roll out of the 6.0 OTA to Nexus 7s after the issue was discovered/reported. I sideloaded the Marshmallow OTA on my N7 only to break the video playback. A friend's 2013 N7 has yet to get the Marshmallow OTA.