Underground Rare Mob

The same mob keeps going underground in my maps... it looks like it has four arms and holds two bows... I'm playing on a monk build with Herald of Ice, I'm not sure if that has anything to do with the bug but I have had the same mob bug out and go underground in 3 different maps... Since it is not on the surface I can't do damage and since it is a yellow mob I can't complete the maps...

I have a screenshot of the incident but I don't know how to post that screenshot here.