This has been an issue since pre-patch, and has been documented by Skyhold, the warrior discord server.
On dummies, Brutal Vitality continues adding to the Ignore Pain absorb even after hitting - and consequently dropping - a capped absorption amount. Reminder that the cap is 30% of max HP.
On any sort of PVE content that isn’t hitting a tanking dummy, this does not happen. Once the cap is reached (again, 30% of max HP), it stops adding to the current absorb on subsequent passes. Again, this is after getting hit and dropping the cap. Warriors have to have IP fade completely for Brutal Vitality to work again as maintaining it after hitting the cap completely stops the talent as described above.