So far this issue appears to be limited to NAS’s with an Apollo Lake processor.
As a stop gap, could those affected please try the following;
SSH into your NAS and then remove the new Intel Media Driver (iHD)
rm "/volume1/@appstore/Plex Media Server/lib/dri/iHD_drv_video.so"
this will cause PMS to fallback to the older Intel VA-API driver (i965_drv_video.so) for HW transcoding.
please let us know if this resolves your burning subtitles issue.
