When a surgery on a pawn fails, damage can be stopped/reduced by the apparel they are wearing rather than applying directly to their bodies. As a result, a pawn wearing heavy armor can suffer no damage from a failed surgery because their armor protects them.
This can be easily reproduced by equipping a pawn with completely new set of power armor and a power armor helmet and having a pawn with little/no medical skill attempt to remove an organ. A message will appear announcing a failed surgery and the metallic tink of armor taking damage can be heard. Inspecting the pawn's gear, you can see that the armor has taken damage from the failed surgery.
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Can confirm. I've been wondering where that tink after putting in some bionics came from.