that's not correct Izaqyos, and, if true, a bug.
however i suspect the possibility of some form of user error - are you sure that the Arch Theurg who cast gateway actually went? that the army was at that time under his command? I'm just thinking that if you teleported in a few arch theurgs you may have confused something - like changing a magic item out of the AT who was to cast gateway, thus invalidating his orders, or some such.
unhappily, since domII doesn't keep any past saves, there's no real way to check
one other thought, the target location didn't have a dome of any sort? nor was there any message about losing an AT (to arrows or disease or whatnot)?