Something in 3641 made the enforcement of unit restrictions a little more aggressive.
UI mods used to be able to circumvent unit restrictions and build banned units anyway. They still can, but now any unit that's not supposed to exist just blows up as soon as it's finished, so it's not much help any more.
Unfortunately it seems there were a few weird edgecases like this and Cybran buildbots (which counted as air units so would keep blowing up). Will tweak it