Mod​(System.​Numerics.​Complex,​System.​Numerics.​Complex)

There is no one remainder of a complex number by another: which multiple of the
divisor to subtract is a choice, and the two usual ones -- rounding the quotient to
the nearest Gaussian integer, or truncating it -- disagree. The interpreter declines
the question and leaves a % b unevaluated; the compiled path has no way to
return an unevaluated node, so it answers NaN, which says the same thing.
For real arguments this is the runtime's own %, and so takes the sign of the
dividend, which is what Evaled does as well.