That’s actually a fairly common behavior: Some newbie shows up and drops some turds, we mods all say “keep an eye on this one”, and then they just disappear, rendering any further moderation moot.
For that matter, making one post and then disappearing is quite a common pattern for non-troll newbies, too. Always has been: Back in the days when it was easy to collect such statistics, our mode number of posts per user was 0, and the median was 1.