Thanks for the info on the system.
Plex Media Server 1.32.5.7349 added support for AMD GPUs, but only for SDR media.
It also added the ability to manually select the GPU in Settings → Transcoder + Show Advanced.
Plex Media Server 1.32.6.7468 adds support for additional AMD GPUs.
The log files show attempts to transcode 4K HEVC video, which, given the file name, appears to be HDR.
Can you select the GPU in Settings → Transcoder?
If not, consider trying the latest beta which supports additional AMD GPUs.
Also, test transcoding using 1080p (or lower resolution) SDR media, whether H.264 or HEVC. Plex does not support HDR media w/ AMD GPUs, and the specifications you linked state “Playback of 2160p videos is not supported.”
