Thanks @JammyDodger for the repro 
Yes that’s happening here - and is one of the issues.
A second issue is that (strangely only some) of the users are reporting that they can’t change their notification levels for a group - whilst it changes temporarily, when refreshing the browser it reverts.
And a third issue is that even if they haven’t manage to change permanently, they seem to be getting the default notification level for the group in any case… (it sets new PM topics to that no matter what their personal setting for that group says)