It's not a bug, this happens in the PC version as well. I personally like to *strategically* hit lvl ultra in the throne fight by going through CCC and finishing palace with just enough rads to hit level ultra using the rads from t1 and/or the pillar guardians
Maybe instead of having the collision mask change when taking damage, you could set a “hit cooldown” variable that resets to a certain value and increases over time, and will only allow health to decrease if it’s above a certain number? Just a thought, I hope this helps! :)