No output is what I was expecting given your previous descriptions. What is happening is Plex is populating the left-hand side of the add folder dialog with the list of the mounts. However, the above code indicates that within the jail, it is told about 0 mounts. This appears to be a jail option according to https://www.freebsd.org/cgi/man.cgi?query=jail&sektion=&n=1 (see enforce_statfs). This does not affect visibility of media or anything on the filesystem; only which mountpoints the jail is told about.
One would normally expect / to be a mounted disk somewhere, but it is not in the list. For reference in my jail it outputted:
Found /
