Another conundrum for you to look into if you would…..
At 17:34 today I posted a reply (message #6) in a Private Discussion. At 17:54 I received email and community notifications that the other person posted a reply, but both of these linked to my message #6 and there were no later replies.
At 17:57 I received another pair of notifications that the other person had posted a reply. These correctly identified the author of the reply, and linked to the correct message (#7)