I think I have the clue.
I've had a look at other rolled up attributes around the directory. Some expanded, some didn't. Probably rules out OS and Java. I haven't got another handy directory to test.
None of the (ou=user) MemberOf attribute lists would expand.
Everything else did...UNTIL I found a rolled up attribute at the BOTTOM of the attribute list. Checked out more. Same, roll ups in the last position wouldn't expand. I'd say that's it. At a high probability
BTW I have two other funnies, I don't think these would affect attribute-level issues, but just in case:
1. I get an error dialog saying "Reading Children / Error while reading entry / Unprocessed Continuation Reference(s)" on first browsing the tree. I've found references to this but not in DirStudio, eg http://confluence.atlassian.com/display/CROWD/2.2.2+Configuring+an+LDAP+Directory+Connector saying that a java.naming.referral option is required. I can't see how to set this in DirStudio.
2. The directory is a big corporate AD, guess, 10k users, 2K groups . DirStudio only browses the first 1000 entries at any node but more can be found using searches. I can't find how to bump this limit, so my browse is typically not complete.