I have noticed that the ACU has two building circles (one isnt actually shown) and have always noticed including gpg. The engineers dont face the same problem either so i know its a acu problem. Anyways the bug goes like this if you just spawned you give your acu the order to build your factory wherever doesnt matter but its in your buildrange and it'l do it fine but the moment ur acu moves even a tiny bit its like the engineering range no longer counts and will have to get much closer (about the same as the circle for the minimum tml launcher on acus).
So if your acu just built something its actual building range will work but if it moves after building or just any sort of action really not involved in building it can no longer use the engineering range and must walk really close to whatever it wants to start building and then it can once again use its real engineering range as long as it doesnt move after.