checked it out on Live, Baron Yosig does indeed flee at low health on Live:
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i noticed on my way to the crypt, though, that the bugs at NG flee at low health on Live.
also, the undead frogs in the crypt flee at low health.... i don't recall if they do that on P99 or not, though (been too long since i've been down there)
as a side note, apparently you can see actual faction hit values on Live now:
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the frogs at the entrance are worth 1 positive Legion faction, but the crypt frogs are worth 5:
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might be useful for some data mining