Okay the fact that it's one of the secret note spots and not just an artifact spot makes a lot of sense. I've managed to reproduce it, specifically by not being the host in multiplayer. Doesn't appear to happen as the host or in single player. It is very consistent though, which is strange. The code *looks* robust, so there's something messed up in the netcode. Looking more into this, and thanks for reporting!
The host doesn't need to have read any secret notes, and this can even occur multiple times! If the players sleep, bombing can produce a treasure chest again. This is very strange.