We use Discourse at Nextcloud a bunch – so first thanks a lot for that great open source software.
A colleague just said that I should look at a specific thread that they @mentioned me in. I let him know I didn’t get a notification and only after some time we found out the reason was that it was a private message.
Hence it would be cool when a person who @mentions someone in a private message who is not part of that conversation gets prompted if they want to invite that person. Of course not automatically invite them, but a nudge would be a good UX improvement.