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I've seen that issue in the past during the quest "The Coral Heart" where tearing down the molds at times invoked a skeleton, which could not be targeted nor dealt damage... though last week within a dungeon (can't recall which atm) there was several skeletons that could not be targeted, oddly though only from certain angles.
"I'd rather be insane in a sane world, than sane in an insane world!" ~Me
Warning - This is a spoiler and looking at it for too long may cause irrecoverable eyesight issues.
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