I was playing a built-in map. I think it happens every time I deploy a full stack, and then attempt to deploy another stack in front of that one. I can't seem to influence whether it's at the positive or negative bound, so I can deploy the weakest army possible or the strongest army possible and leave the base with the opposite kind of army at the garrison.
Sometimes it happens to me even if I haven't already deployed a stack, not sure how that happens but it's a rarer issue than trying to deploy a stack after I've already deployed the maximum army