I’ve just come across this. Currently it seems that only staff can use the bulk PM actions. However, the option of selecting PMs is also displayed to users but once a PM is selected no actions appear. I’m sure that in the past there used to be an option to archive / move to inbox when selecting messages as a user?
Looks like when the feature was added to allow normal users to selectively dismiss topics on /new or /unread, this condition was added to the bulk-select button:
canDoBulkActions: reads("currentUser.staff")
…and that’s preventing it from working for non-staff in their inboxes. We’ll get it fixed!