I believe it’s not an isolated issue. I’ve had threads where posts by spam bots that were flagged and deleted a couple of years ago had resurfaced after the forum update. And there was at least one case where a post by an MP customer that had glitched out (he contacted me via mail due to that) at the time of posting, became visible after the forum update.
I had noticed both these cases right after the change to discourse. Since it was just a handful of posts, I deleted the irrelevant ones manually. But I’m guessing it’s the same issue that you encountered here.