Oh, shoot! The misseds secrets are in the Cavern, not the Chamber.
Ah, they've probably already selected an action by that point. I'll look into this.
It turns out I've placed seven 1-HP life gems plus a 2-HP one in earlier areas, locked behind the Raft. I'll sneak in one more next patch- it'll be locked behind the raft, but should be pretty easy to find- so that the max possible HP is once again a nice clean multiple of 5.