I have been playing around the 1.16.2 pre-release. I was trying to find a Treasure Room bastion to look at the improved loot. I teleported myself to bastion after bastion, but not a single Treasure Room. I don't remember treasure rooms being super rare before, and there's no mention in the changelog of them becoming super rare.

I am not kidding when I say I teleported to 40 different bastions, and not a single one of them was a treasure room. It could be very bad luck but the fact I visited 40 and saw not a single treasure room has led me to believe this is a bug. If each of the 4 types has an equal chance of spawning, the odds that I'd not find a single one out of 40 is almost nil.

I must also add that a disproportionately high number of bastions were the "bridge" type - at least half of them in fact.

EDIT: Visited 100 bastions, no treasure room. Absolutely certain this is a bug.