In our (private) forum, staff participate in casual group PMs much like any other member. So there’s nothing special about a staff PM to a group in this context – except that it will push through to suspended members, contrary to expectations.
To compensate for this, we’ve been removing suspended members from all groups. It’s an extra step, but it’s better than accidentally PM’ing a suspended member and having them continue receiving emails.
Think about this in the context of suspending a member for harassment or personal attacks. You wouldn’t want them to keep getting these emails just because a staff member is involved in a private group discussion.