it is very likely that you need to give the Codecs directory in executable permissions
Plex (for some convoluted and very inconvenient reason) does not include the transcoder needed to convert TrueHD and EAC3/DD+. instead it downloads the required files and executes them on encountering a media file where it’s required.
depending on how your permissions are set on that zfs dataset it probably can’t give the file execute permissions from the container (this is a GOOD thing, leave it that way), you should just chmod +x the codecs directory from outside the container.