Might be some files didn't get installed properly, HP per level changed, this needed a certain boost to be given to mites so they would not auto-die... Meaby that part did not install on your computer.
Try explaining what faction you were playing, meaby give a screenshot of the lairs, and also monster density settings and world difficulty.
World difficulty might have a thing to do with it, I know they have double hp on ridiculous, so might be some easier levels just kill off all the mites to help you get more free loot 
Sincerely
~ Kongdej