This is most likely just an error, they possibly may have created a post in the past and deleted it to quick for the website to process, then it changed it by -1 even though the post was not counted to their stat’s yet.
Not really sure if this is a bug on my end, and if only this account had it. I’m also not sure if you have fully solved the problem, but I will mark this as the solution.
I’ve seen multiple occasions of times where users have their only posts removed, which leads to showing negative posts created.
Very common bug, in this case it was just manually patched.
This is pointless to put time into. Discourse re-calculates statistics from source data every week anyway.
Seems like base Discourse has some issues where negative post count can occur. They’ll probably get auto-fixed over time as they are reported to Discourse.