I'm playing on a server and saw this message come up twice in a row. There are several people online so I have no idea what others are doing although several minutes before, a couple of players got [He Wasn't So Strong] so they may have still been working with bosses (I was AFK in a browser). The players DID get "xyz was slain by Herbert" and "...by Sharon" before they got their achievements. I'm not an admin so I can't check recent logs sorry but I thought I'd mention it.

We're running Apotheosis-1.16.3-4.4.1

That's rather interesting. For that to happen something would have to have caused execution damage sourced to a player, which should be impossible. I can write in the translations for it, but just weird nonetheless.