If you open the belt in a delve to select a new effect, you won’t be able to open any of the treasures (chests, rare loot, mushrooms, etc…) throughout the delve. You’ll get a “You have an unselected choice already. Please select a choice to open a new one.” message, despite not having anything actually pending.
I’ve only tried this in Fungal Folly so far, so I’m not sure if it’s just this particular delve, but regardless, should be looked at.
Edit: Just tested in Spiral Weave on a different character. Same issue.