I have two roles with mutual exclusivity using policy rules and enforcement.
If I assign both to the same user, the policy violation message appears after saving the user in the user list. So it seems like the request was saved.
The same situation with checkbox Keep displaying results will not allow to send the request!
The original (now deprecated) role exclusivity does not allow the request to be sent.
The behaviour should be consistent, for role exclusivity with enforce it should not allow sending the request.