There isn’t really a way to know.
We know Hulu has an app supporting both HDR10+ and Dolby Vision on FireTV, and they serve these kinds of files containing both.
As for how they handle the decoding and stuff, it’s impossible to know.
Maybe Amazon would be willing to help Plex if they reached out and explain how they and others are doing this.
I do what I can and, as always, push it to Engineering. That’s how I’m told to handle issues.
If they don’t reach out to amazon, what can I do?
I can try one other thing. Maybe that will effect some progress but it might not be pretty. 
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Thanks @ChuckPa and good luck!
As an aside… the majority of UHD movies I’m adding these days are both HDR10+ and DoVi P8. The Fire TV is a pretty much unusable device until this is fixed for anyone in a similar boat.
I’d hope that a CCwGTV would be solid alternate device, but it currently has the issue where it can’t play AVC files with 4 Reference Frames which is all Blu-ray backups. That means the CCwGTV is unusable for me too since that describes all my FHD files. It also has the same issue as all Android devices (Fire TV included) with currently being unable to play when there are multiple versions if an audio transcode is necessary.
Maybe a Roku? Nope, because it doesn’t support PGS subtitles, and the majority of my movies are Blu-ray backups with PGS subtitles… many with Forced subtitles for foreign language translations.
Only thing left is the Apple TV, but it currently has an issue with choppy UHD HDR video playback for some movies.
I personally am currently using Infuse on Apple TV, but am looking forward to being able to their being a streaming device that I can easily recommend to friends/family again.
I love Plex and am a diehard user, but this is becoming problematic 
Would you be able to sideload this build (if you can see) and see if it solves your problem;
FireTV / Armv7: https://artifacts.plex.tv/android-experimental/9.17.0.38689-2727c07f/PlexForAndroid-9.17.0.38689-2727c07f-Armv7aAmazon.apk
If not, would you be able to provide logs after reproducing the issue?
Hulu won’t be serving HDR10+ and DoVi in a single stream.
I’ll let quietvoid chime in on this too, but I thought Hulu was possibly one of the few (maybe only) streaming services that actually serves up their HDR movies with HDR10+ & DoVi Profile 8.1 metadata simultaneously.
I wouldn’t of thought so as it would increase their bandwidth costs slightly (with a lot of traffic that would add up). But without poking at the stream itself I can only assume.
That doesn’t make sense to me either… Why send both streams when you only need one? (discarding the other). Isn’t that 2x the data ?
The problem persists with that build. Here’s the log: plex.txt (82.2 KB)
Also as a note, with all players it is impossible to get any UI elements when playing the video.
Judging by the amount of media available out there from Hulu, some titles are definitely HDR10, along with HDR10+ and Dolby Vision in the same bitstream.
Whether this is what gets served to the app on devices, I don’t know.
Does the sample contain a HDR10 fallback? Its coming through as DoVi P7, which the FireTV doesnt support so its using the standard HEVC decoder.
Yes it has HDR10 fallback. The file triggers Dolby Vision fine as it’s profile 7 MEL.
I can also test with self made profile 8 files but the result would be the same.
The 4K Max is particular in that it doesn’t need hints to the decoder to activate Dolby Vision, so even if it uses the regular HEVC one, it can use the metadata within the bitstream.
The only devices that support DoVi p7 MEL are the SHIELD and Chromecast with Google TV, the FireTVs dont support them which has been fixed in the build I shared earlier to prevent just playing a black screen.
It does seem like a device issue, but I doubt it would be something that Amazon are interested in solving as their DoVi capabilities are meant for streaming and not personal media. So I will look into whether or not we can strip out the HDR10+ metadata before we pass things to the decode.
I don’t know about FireTVs themselves, but both 4K/4K Max sticks and 2nd/3rd gen Cubes support profile 7 MEL just fine.
The workaround works fine for the 2018 4K stick, for what it’s worth. I use an old firmware, though.
The 4K Max doesn’t care about it, it just goes into Dolby Vision. 3rd gen Cube as well, probably.
So the only solutions are for Amazon to fix it, or straight up ignoring parts of the bitstream.
Hmm the FireTV Cube 3rd Gen I have here does not support DoVi P7, neither does the FireTV 4K Stick (I dont have the Max so cant confirm that), playing a DoVi p7 file and it shows a black screen, disabling DoVi p7 support though and the file plays as HDR10 without any problems.
Maybe the files you’re testing are actually FEL? Those show a blank screen as well.
It’s not possible to distinguish the two from ffprobe or MediaInfo info, though.
That is unless you were to get the metadata for the first frame.
@sixones & @ChuckPa, as @quietvoid stated, Hulu would benefit from this because sending a HDR10+ and DoVi metadata in the same bitstream would result in them not needing to store multiple copies while also not increasing bandwidth due to how tiny the metadata is (keeps things very efficient).
Here’s a 30-second sample file with both HDR10+ and DoVi Profile 8.1 metadata from the intro of “The Invisible Man (2020)” (none of the actual movie is in this clip). This movie was released on UHD Blu-ray with both HDR10+ and DoVi metadata in the same bitstream on the same disc (with HDR10 fallback) (the DoVi metadata in this sample has been converted from Profile 7 to Profile 8.1 as that is the most common format for Plex libraries these days (thanks to @quietvoid).
Technically they are separate on the disc, they only become the same bitstream after ripping with MakeMKV.
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Good point. Edited my message above.
No Android device supports FEL, we can only play the MEL layer, the enhancement layer is ignored.
Profile 5 is the most common format, followed by Profile 7.
Why convert from profile 7 to profile 8?
I’ve tried both of the samples on my FireTV Cube 3rd Gen, and they both playback correctly with no black screen.
The first sample with DoVi p7 and HDR10+ plays back as HDR10 as expected since the device doesn’t support p7. The second sample with DoVi p8 plays as DoVi.
Here is the APK I used if you want to give it a try; https://artifacts.plex.tv/android-experimental/9.18.0.38760-5b1566a4/PlexForAndroid-9.18.0.38760-5b1566a4-Armv7aAmazon.apk
