Well, found an answer, but as it’s from 2019 I don’t know how current it is. The pull request below was the one that introduced the Bone Seer into the game, and in it someone explicitly states that chance does not work on place_npcs. Looks like the main way to get an NPC ‘random encounter’ is via nested mapgens, which I have to look up more details on.
Still kinda weird, as it takes the parameter and doesn’t spit errors back, but I know next to nothing about C++ so…