Zz'Rot Portal: Voidspawn's health decay?

I've tried out the portal a few times and it seems like if the voidspawn's walk more than a few steps their health starts to decay rapidly and they die before ever doing anything. I don't see anything in the item description or notes about this being a mechanic, and I've noticed a few other players placing their gates far down a lane only to have their voidspawn's die before doing anything. Anyone know if this is intentional?