This is a recreation of
NAS-108197 since it was improperly closed due to a misassessment. I would reopen it myself, but I can't.
I'm trying to access a folder of a mountpoint as user www. The folder permissions are set to 2770 (drwxrws---), which has read, write, execute set for both user and group, as well as setgid set, and are owned by an unregistered user (from the jail's perspective), and by testgroup. www is a member of testgroup.
NextCloud% ls -l
drwxrws--- 4 1002 testgroup 4 Aug 17 2018 testfolder
NextCloud% cd testfolder
cd: permission denied: testfolder
NextCloud% id www
uid=80(www) gid=80(www) groups=80(www),972(testgroup)