The function wiki_user_can_edit right now has this behaviour:

If a wiki is collaborative with visible groups, a user can edit a subwiki if he belongs to the subwiki's group.
If a wiki is collaborative with separate groups, a user can edit a subwiki only if his active group is the subwiki's group.

I think the behaviour should be the same in both cases. Right now if a user belongs to more than one group in separate groups then wiki_user_can_edit might return false when it shouldn't.