If it is at the beginning of the mission then its not a bug. The first slave maker will be ignored, just telling you that they now found you and that raids will get started. Only when the waves are starting your ants will fight the slave makers, at least thats what i figured out
If another wild ant hit your ants when you have them set to gather and not fight they will fight back, but only the soldiers where as the workers will ignore it I think.