Plex should work for profile 7 on the Shield but it is far from reliable. There are many times it can take 3 or 4 tries to trigger.Question #2: Any MKV file played on the shield using Plex would only display as HDR10 rather than Dolby Vision on my Sony A80J. All MKV’s were made on v1.15.2 to v1.16.5. I confirmed that other DV sources work (such as Disney+). It seems others have had luck with playback of varying DV profiles using Shield Pro including profile 7. I had no luck with any profile 7 files, I was curious if this is typical or an issue only on my end?
I highly recommend Just Player for playing DV on the shield.

