I had this happen and noted that the system was devoid of inhabitants. Was able to decolonize after I shipped in some unwanted pop from another system. May work for you?

Hey there :)

It's not a bug! If you conquer a system by force, you don't have full ownership of it from the get-go.

The population is still loyal to the old government and will need time to progressively adapt to being under your reign :)

Cheers,

Ok well, that makes sense. But As the primary use of the de-colonization tech is to stay under the planet-cap, wouldn't it be better to make it straight up available at all times on all planets?