It should be a permissions issue if its allowed in PMS.
How are the files exposed to PMS? I’m amusing you setup a mountpoint in the jail can you post the ownership/permissions of the dir/dirs where the files are?
This is mapped to the “freenas” user, PMS will run in the jail with user “plex”, you need to either apply ACLs (usign getacl/setacl in the terminal) or just change ownership to plex
wait is that all data in the pool? Its probably better to have a dataset for “media” or “plexmedia” unless you don’t care and just want to give “777” to all
But if the idea is for PMS be able to read and delete media on a gives path then then the best way would be to keep the media in its own dataset and then mountpoint then set user/group to plex (also tick the “recursive” option)