Using forum in visible group mode I find, that it can't correctly handle groups in 1.9 ( bug preserves in current version of Moodle 1.9+).
'Group' column is wrong and combo-box too.

On the attachment you can see fragments of two pages: one from the forum (it tells, that group of the student is FEVT-1.1S), another from the user info (link from the forum page) that tells, that group for this student is FEVT-1.2S (which is correct).