Good question indeed.
As I said before, every time resistances are applied to the player or a monster, the code calls a method that requires to specify the new cap. When you anger Mystera, this method is called with a new cap of 100.
At that points, two things can happen: If the monster already has resistances, then the new cap becomes the
minimum of the current cap and the new cap. So the cap can only go down.
If the monster has not this type of resistance already, then the new cap is directly applied.
This should not have much impact, as most enemies that start with resists have a 100% resist cap, but not all.
For instance, the last form of Anoobis in Creeplight Ruins seems to have a physical 75% resistance cap.
Also it made me realize that many monster in the Extreme Edition have 90% resist cap instead of 100%,
but we might change that in the next release
For instance, if you anger Mystera a lot so that Stheno gains 100 resistance, and then you attack Stheno for the first time,
her power should set the new cap to 90 again!
The effective resistance that is displayed always corresponds to the actual resistances used for damage computation,
so there is no reason to worry about that.
Also, even if a monster has 100% resistance, you can still damage him with corrosion.
So if anyone wants to try a Rat Monarch of Mystera run in Hall of Steel or Vicious Steel, it might work.
@maya: did you win with your Assassin of Mystera in Hall of Steel? Did you reduce the boss resistances again first?
Because if you keep angering Mystera beyond 100% resistance, then the hidden resistance count of the monsters
should keep increasing, and then her Weakening should have no real effect.