When an admin splits replies away from a thread for whatever reason, that admin shows up as the Most Recent Poster (when they are not, new replies have been made since their split).
I first noticed this December 13, 2020 at 1:09PM EST.
It’s not a bug, it’s how discourse works. Technically they are replying to the thread, just for merging things or whatever admin tool they are using. This happens every time when they do an action like closing, unposting, merging, etc.