It happens here on Meta and also on my forum (just updated).
@ followed by a group name, the auto complete pop up shows only username and not groups name
I’ve tried it with both moderators and admins, I do not see anything
The auto complete works if I try with custom groups if the name is composed only of one word (it works for
translators but not for
theme_authors and so on)
Maybe the bug is related to this discussion Group long names - help give full citizenship to groups for mentions, messaging, group pages?