It does stack, but its not additive. Iirc, this has been covered in a different thread before and the devs themselves explained it.
If memory serves me right, the odds are "additive" after being multiplied to the past %. For ex.,
Cast party gives you a 50% of Tag Ins being UNBLOCKABLE. If you have 3 peacocks with maxed out Cast Party, you get the following:
50% (1st) + (50% × 50%) (2nd) + (50% × 50% × 50%) (3rd) = 87.5%
"Abilities in the game doesn't know that similar abilities other than them exist." - from one of the Dev from the mentioned thread above (its just what I remember them saying)
And this works for the other abilities in the game as well. They follow the process below:
(Event/Odds 1) > (Event/Odds 2) ... > (Event/Odds n)
So what actually happens in-game with 3 peacocks with max cast party is that each tag-in's odds happen one after another, rather than all at once. So 50% Unblockable > if no, 50% Unblockable > if no, 50% Unblockable.
I hope that cleared things and I hope I did my math right on that one lol