So as a reader, you could actually theorize what's happening here, but none of the characters have all the peices of the puzzle, so would not be able to figure out why this is happening

Ah yes, a classic case of dramatic irony

Hopefully it'll be explained how that group wasn't affected.

Alas, Poor Merricael, having to put that charade facade up to keep the truth hidden.

Took me way too long to realize: they're all mindlinked, and in particular Merric is mindlinked to the rest, so they have his "protection."

Dia, etc. don't realize that Merric is naturally immune, and Merric doesn't know about *all* the mindlinks (nor, I presume, the details thereof).

Magnema wins the prize