I've found that another nation simply can't take the 3 tiles that are at the middle of a fort. If I try I can get all the other tiles around it but not the 3 fort tiles themselves. Even in a split I can't take these fort tiles. Is this a bug, a feature, or a dumb mistake on my part?

same, sorry i cant help but a guess is

the forts are like huge castle walls surronding towns, and lots of millitary are concentrated there, so its almost impossible to break in, but again its prob a glitch, no way my theory is true

If you try with the largest brush, it somehow works and conquers it very easily. The middle and small brush doesn't work on that