Undesireables/Displacement - Not working or going down in pop #

Conquered half a rival's stars next door and ended up with two of their planets. I set the species there to undesireables/displacement in order to drive the planet to uninhabited. Works normally, but in this case, two of the three species stopped dropping by 50 or 100 per month and now there's just a - symbol there.

Any idea what's going on with that?