Answer
This expression style has been observed to only behave reliably when every user actually belongs to at least one user group — if some users belong to no group at all, the "not in group X" condition seems to evaluate as true for them regardless of intent, effectively treating an ungrouped user as matching every exclusion; adding a catch-all group (like "ALL") that every user is a member of makes the not-in-group-X expression behave consistently for everyone.