Should be fixed in 1.40.4.8679 for PMS on Synology.
I’m running that version on my DS918+ (J3455) and hardware accelerated transcoding and tone mapping are working. PMS installed via Synology Package Center. No Docker, etc.
Plex Media Server 1.40.4.8679
FIXES:
(Tonemapping) Hardware Tonemapping failed on intel devices when run on Synology (PM-1667)