I build a Dallad deck using 3 copies of Iddi
It saved fine and showed as valid in builder
When I tried to play it in a game, i got a popup saying deck was invalid and confirm to go to deckbuilder (that really needs to say WHY the deck is invalid)

I took out hte 3rd Iddi and it worked

I believe that the builder is reading the Dallad exemption for Prismatic cards but the encounter is not.

I expect they already know but I gather it's not just Dallad and may be across all mercs that break the grid