I'm getting 2 turns for immunity talismans in the arena which is weird, but maybe to do with the Prisoner's passive? As his passive only works for half the turn anyway.
Might have to try it without the Prisoner, and also see if his passive effects any other talismans duration...
Edit - Without Osiris its 3 turns but with him its 2. So is that intentional or a bug? His passive turning regular immunity (3 turns) into perfect immunity for 2 turns....