HW Transcoding Not Working for HVEC/H265 Content (1019+)

Plex Media Server NAS & Devices
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It may be related to this:

From what PMS version did you upgrade? (Some) versions 1.32.5.x and below worked fine. Also, try disabling HDR tone mapping in your Settings → [Server Name] → Transcoder settings. That seems to be the catalyst for some failures.

Regardless, you probably should post in the linked thread with the information requested.