Summary: I loaded a world with Caves n Cliffs Experimental Toggle Enabled and found out that the bonus chest was 50 block beneath my spawn point in a massive cave.

Steps To Reproduce:
Step 1: Load The World Creator.
Step 2: Enable Caves n Cliffs Experimental Toggle.
Step 3: Enable Bonus Chests.
Step 4: Locate Bonus Chest.
Step 5: Repeat.

Observed Result: Bonus Chests can Generate in Caves.

Expected Result: Bonus Chests shouldn't be able to Generate in Caves.