I think I may have found a bug regarding the occupation mechanics in Terra Invicta:
https://imgur.com/zwRXq4L
I went to war with the Alien admin, which had popped up in the USA. My first armies landed occured at New York, and my forces sieged down the region completely. Because the USA was at war with me and controlled by the servants, the territory was never returned to the USA.
However, eventually, I changed the USA to my control. As I went around liberating the rest of the alien admin regions, they all returned to the USA. All of them except for New York. Now, the alien admin has changed its capital to New York, but I cannot siege it down, because the game seems to think the region is already sieged down. I am not sure how to correct this issue.
Removing the units currently there and putting them back in does not fix the issue, nor is there an option under policies in the EU to return the land to the USA.