To reproduce, go to, say, Group forum permissions for the Guests (or any) group and assign a role, say Read-only.
Then, using Advanced permissions, change on permission in that role. As expected, the assigned role now reads "No role assigned".
Then, using Advanced permissions, go back and revert the one permission you changed back to its default for that role.
I would expect that the ACP would now report the original pre-defined role, since all the component permissions are the same. But it does not - it still shows "No role defined".
If you go back and assign the defined role, then it does report the correct role name.
One would think that if a set of Advanced permissions is exactly the same as that of a pre-defined role, it would be reported that the permission set IS that pre-defined role.