If I send a PM on a (different) forum that uses Discuss and get a reply I don't get notified in the Welcome back block top right.
Is this a current feature / bug ? I think they may be running an older version as I get notified I have a new PM when I send one, which was a reported bug now fixed AIUI.
I don't get that notice on this forum for subsequent posts to PMs, only for the original PM, so I would presume it's a common situation to Discuss in general.