I would like to thank you very much for looking into this! 
The post count change certainly does help, in that the topics and replies are both considered in the count now, so that took care of some buggy behaviour, excellent!
I must be doing something wrong though, if I take the following steps:
- Register a new account manually (not through the admin interface)
- Post several topics through that new account without logging out
In this particular case (and it is a particular one ), the welcome dialog continues to appear no matter how many topics and replies are posted, at least in my testing. It does disappear once you logout and log back in, though. I think the reason I was seeing it more often than before was due to the bug you have fixed here, so it’s certainly improved upon the situation!