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I just installed a Fire TV Stick 4K, along with the Plex app. When I play a DV .mkv movie in Plex, it looks great on my DV-supported TV. However, playback does not trigger either the "HDR" or "Dolby Vision" flags on my TV. HDR movies without DV don't trigger the flag either.

In other words, my eyes tell me DV is working, but I can't confirm this on the TV.

Anyone else run into this situation?

Nope. You should see the mode switch pop up, no matter the input device.
No issue with a C8 and the stick for me.

I wonder if it's an issue with the Fire TV Plex app and my TV. It's a TCL Series 6, the 2018 model.

There's a huge visual difference between playback on the Plex app on my TV and the one on the Fire TV stick.

The Fire stick will only play Dolby Vision if the Dolby Vision enhancement layer is MEL not FEL. Here is a complete list of all the 4K movies and what enhanced layer they are.

https://forum.blu-ray.com/showthread.php?t=276448

Secondly, The firestick is set in the settings as Dolby Vision as default for even the fire stick interface (If your TV supports Dolby Vision that is)

To enable your TV to pop up and say HDR or Dolby Vision you need to change the setting of the Dynamic Range to adaptive, which is under the display & sound settings if I remember rightly.

Hope that is of some help.

Lots of help! Thank you.

Well, half-way successful. After changing to "adaptive", the HDR flag appears on my TV, but not the Dolby Vision one. The DV appears when I stream a DV source, so I suspect its a Plex player shortcoming.

The Plex app should still play Dolby Vision if it's MEL but I think there are bugs with it still.

I've just bit the bullet and purchased a Nvidia Shield Pro instead, that's playing every Dolby Vision video file I throw at it.

You can't use refresh rate matching in both the stick and Plex's settings, otherwise it doesn't work all the time.

Either way, working files would only be profile 7 MEL as mentioned previously.
Other profile 7 files would still trigger Dolby Vision, but show a black screen.