I installed this app. As admin I found the Manage multiaccess section in steup (I use the swedish installation so do not know the english terminology). I ticked for Use the extended permission feature for managing locks. I locked some files in a common folder for all users.
I logged in as a regular user with no admin rights. I could open and delete the “blocked files”.
I use Nextcloud 12.0.4
By “common folder” do you specifically mean a Group Folder (created/shared via the administration interface)?
I installed the app in NC 12.0.3 and did some tests. For a workgroup test I added an NC user which only feature is to share folders for a workgroup. Currently one folder is shared with 2 “normal” users. The I logged in via web interface as one “normal” user ans locked one of the shared files. As the other “normal” user I was able to manipulate the “locked” file via NC client sync and webdav. The web interface shows that the shared file is locked but the lock seems not to work. I was also able to unlock the file via web interface as the “other” user who did not lock the file.