In moderated rooms, XEP-0045 says that roles should be persistent across visits.
Roles are temporary in that they do not necessarily persist across a user's visits to the room and MAY change during the course of an occupant's visit to the room. An implementation MAY persist roles across visits and SHOULD do so for moderated rooms (since the distinction between visitor and participant is critical to the functioning of a moderated room).
This is not currently implemented in mod_muc. I would be willing to work on a patch for this with some help.