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I know there have been various topics around vanishing enemies. I have the same issue here and I can now manage to replicate it every time which I thought might be useful in helping to squash this bug if it hasn't been squashed already.
How to replicate: SE of CHARNAME is the girdle Ogre. Fire a couple of arrows at him to get him chasing and then head W and N when you can. What you are aiming to do is to lose him in the fog of war just past the natural barrier currently above you as soon as you can head N.
You need to keep him quite close so he should be just caught in the fog of war as you get around that barrier.
Hope that helps.