Yes. Posts aren't actually deleted, they are just flagged as "don't show". Any "deleted" post can be easily restored by an admin or host.
With few exceptions, Aksimet is entirely responsible for such actions and there is no human involvement. Sometimes users report spam threads/posts that Aksimet didn't catch, so we have to manually get rid of them. More often, posts go away and we have no idea why. Aksimet works in mysterious ways. It does not tell us its reasons.
Whenever I come across a post deleted by Aksimet, I open it to see if there was any reason for it to be removed. If there isn't, I just quietly restore it. I do this several times a day, as do all the other hosts. It's a PIA. We wish Aksimet would smarten up, but there's nothing we can do because it is designed to be an independent, self-learning AI.
Despite our complaints about Aksimet's false-positives, though, I have to admit that it has helped reduce the spam problem, which used to be much, much worse. Think about it: when was the last time you saw an obvious spam thread or post here?
No, the thread did not disappear because you added "[Solved]" to the subject line. It disappeared because Aksimet re-evaluated it after the edit, and one or more of its cryptic AI rules had changed since you first started the thread.