Pray to RNGesus.
It seems to work fine for my Mages, so I'm unsure what the issue is with you specifically.
The way our game engine works makes it so rolls really are less random than we think, usually leaving clusters of good and bad rolls is how it works. If this is only happening part of the time, then I'd say it's just rotten luck. We don't really have a lot to go off of here!

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Sometimes when I'm fighting a group of enemies, more of them will appear mid-fight in the same spots that the first group popped up out of. Is this a bug with enemy respawn rates?
A little more, please? Where does this seem to happen and with what types of mobs? I'd wager it has to do with area design, some spawn triggers are really close together while others are more spread apart.