I have all 30 Caves marked on my Vallis map as I've found them all, but I assume one of them bugged out because I happened to be a client during some of my game time. (and there have been quite a few client / host bugs where I've seen info not sync correctly.)
Here's a bit of proof.
Hopefully gets fixed soon.
Back in Update 23.10: Chimera, the Joyride achievement was added and I was quick to earn it in-game, but it was unavailable from Steam's achie