So between me and another player suffering the same bug. him at 26/30 and myself at 28/30 caves and with us both having the same map info and having both explored the map cave systems several times going in and out of each entrance I have come to the conclusion the achievement / challenge is bugged. In the sense that I believe somehow if you join another players game the caves were somehow bugged for the joining players, perhaps during a bounty join which is mainly what I was doing. I am curious how many other players have experienced this as I have seem some other forum posts mention similar issues too. I have attached a copy of my map to this post.
https://imgur.com/a/UGgleCF
EDIT: added another picture into the link showing my challenge status, note that all 30 caves / all entrances have been explored and showing also.