You're correct on both the mummy and ghoul requiring magic weapons and that the arrows need to be magic. I'm guessing that the magic weapon check was done against the bow and not the arrows for some reason. if you have access to a normal bow, try swapping them out and seeing if you get the same results (you do damage) or if the non-magic bow yields the expected results (invulnerable).
You landed a crippling blow with your bow, not a critical hit. I've never played a warrior, but I surmise therein lies the difference.
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