They can be. I made some that works good enought for some modes, but its hard to do it by AI editor made by Blizzard 'couse of limmited AIs at the same time. My PC have trouble late game when I am reaching 10 of them. AI to custom mas just can't be tested by WC3 Editor.
If you want to fix that bug check positions of all structures on the AI list. If order AI to build 2nd stage town hall before getting new goldmine it will make it at 1st town.
You can order AI to build only in the 1st town but there is risk that AI will lock all free space and then can't do any. If you still want to do it just change the destination to #main town (originaly there should be "anywhere")
Look how it should made (red text is translation):
English editor would be realy nice... :/