We "regularly" seem to have a problem with groups of users missing from our LDAP during LDAP-syncs (a completely unrelated problem which will not be fixed soon ). They show up again after a few minutes, however, we noticed there are consequences when looking at their accounts on our Moodle system.
Grades, thankfully, were no problem whatsoever. However, I noticed the participants had been removed from their group assignments, i.e. they no longer were able to access activities which were limited to group members only. Also the grades they had achieved in these activities no longer showed up in the gradebook despite the attemps still being available. It's confusing indeed...
(using Moodle 2.5.1+)