Platform, device version and operating system:
Screenshot or image:
What you were expecting to happen, and what actually happened:
For each rarity of room in Delves, the Treasure multiplier and Chest Upgrade chance (when converted to decimal) adds up to a certain fixed number.
Rare is 1.35, UR is 1.50, Epic is 1.65, Leg is 1.80.
This is true for every single room you can encounter in Delves, including newly-added rooms or ‘Treasure’ rooms – except for Rocky Cavern. It’s an UR, yet adds up to the combined multiplier/upgrade chance of a Rare.
(Maybe the rarity got upgraded in the development process, but the values didn’t?)
How often does this happen? When did it begin happening?
Always, since inception of Delves (or at least since the thread that I’ll quote below).
Steps to make it happen again
For context (Oct '18):