I agree with you. I created a similar MKV file, and have similar results.
With current PMS and the latest Plex testflight tvOS app:
MKV file with HEVC DV P5 now plays!
Direct Stream.
The TV switches out of DV mode into HDR mode.
The Apple TV has excellent tone-mapping, and color looks good.
Dynamic brightness is NOT working.
MP4 file with HEVC DV P5, stored as dvhe, still doesn’t work.
MP4 file with HEVC DV P5, stored as Apple-preferred dvh1, Direct Plays perfectly.
TV remains in (or switches to) DV mode.
Dynamic brightness/metadata ARE working correctly.