Pathing Mob unable to be hit in Befallen

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Started video midway through fight thinking we were gonna die to this:

https://youtu.be/s-YgZkjY29Y

This was the SK PH from room #3 on the wiki, he was body ageoed, some how fell down hole but then popped back up and still hit us but we could not hit him. We were able to thorns everyone getting hit and kill it that way. No XP either since we didn't get to do a single point of damage.