I'm not classifying this as a bug quite yet. I'm still gathering information on it. It might be working as intended.

So there is this thing where colonists on colony ships will return to a planet if you decommission the ship. The reason I discovered this 'feature' was because I was trying to get rid of Festron population.

Here seems to be the rules:
-The ship you decommission can be a colony ship, constructor, or transport. If it carries population, it matters not what kind of ship it is.
-If you decommission a population ship near a core world, it will transfer the population on that ship to the nearby core world.
-If the nearby core world is filled to the capacity with population, the population from the decommissioned ship will be placed on the nearest core world with capacity. This seems to work even if the ship is in a different sector.
-If none of your core worlds has space for another unit of population, then the population held by the ship simply vanishes.