I have the Fire TV Stick 4K Max since a week and have the same problem (Fire OS is 7.2.5.5).
The problematic video with DV and HDR10(+) has the following codec information:
Format/String : HEVC
Format/Info : High Efficiency Video Coding
Format_Profile : Main 10@L5@High
HDR_Format/String : Dolby Vision, Version 1.0, dvhe.08.06, BL+RPU, HDR10 compatible / SMPTE ST 2094 App 4, Version 1, HDR10+ Profile B compatible
CodecID : V_MPEGH/ISO/HEVC
Plex for Fire TV also says that there was an error and that the video had to be transcoded. However, when I open the Plex Media Server it says that it uses direct play (the CPU is also not working).
Another video with just HDR10(+) works:
Format/String : HEVC
Format/Info : High Efficiency Video Coding
Format_Profile : Main 10@L5@High
HDR_Format/String : SMPTE ST 2094 App 4, Version 1, HDR10+ Profile B compatible
CodecID : V_MPEGH/ISO/HEVC
The video also doesn’t play in VLC for Fire. However, Kodi is able to play the file.
The strange thing is that the video plays just fine using the Samsung version of Plex on my 2016 43K6400 or directly from USB.
Someone at the Amazon forum also reported this problem:
Is this a Fire OS problem or can this be fixed in Plex?