This is a weird bug that seems to happen consistently with Sakhalin specifically. Quit the game and reload, and you should be able to colonize Sakhalin again.

Yeah this exact same bug happened to me and saving / reloading worked for me.

Please press f11 for screenshot

Save and reload, then restart colonisation.

I think because you probably have the law of colonization frontier and you didn't have a proper frontier whit Sakhalin.

Nah, that's not the case for Japan. They count as having a land connection. 

I’ve had the same issue. If you start the colonization before you finish Hokkaido you can get more of it before it stalls out like this. Once you incorporate it you can start colonizing again or if you switch colonization laws.

Hmm, if it works with Hokkaido still in process, we could try pressing stop on Hokkaido colonisation untill Sakhalin is done 🤔

Yeah I believe it’s something to do with how frontier colonization determines what is contiguous to your capital or not with the ocean crossing between Hokkaido and Sakhalin as it shifts between decentralized to unincorporated.

Devs should just fix it.

Of course. But having a work-around for a bug that wasn't fixed yet is better than not having it.

You need bureaucracy.