FIGHT attacks them anyhow (useless, because they can't damage the target). It seems to completely ignore that I am canceling the command, in fact

I've tested, and it appears that when PATROL is set, this doesn't seem to cause the problem. If they are set on MOVE, they also continue to engage only valid targets, while moving forward.
It would appear that FIGHT not only keeps attempting to issue ATTACK commands every gameframe, but that somehow it's not being canceled. This leads to the irritating spectacle of watching Units that have to deploy (i.e. stop moving to fire) getting "fixated" on things they can't kill. For now, I'll just disable the FIGHT command entirely, but I wanted to make sure that this bug was reported.