there is an allied computer player with campaign area town AI. I have set some of the units of that player to order: move to location. At first, the units follow that order, but after several seconds, they start returning to their own base, even if i use preserve trigger.

Well AIs have their own orders for units.
But preserving the order should work. Are you sure you always have the location from which they're ordered centered on these units?

OK, I didnt move location, that was the problem.