It's something that came with RnF I believe.
Used to be that you can form national parks using tiles outside the 3rd ring as long as the city owns it...
Which itself used to be buggy in my experience; if you formed a park where all 4 tiles are outside the 3rd ring, the park is formed and amenities are awarded, but tourism and # of tourists, at least according to the lens, is zero. If at least one tile of the park is within the 3rd ring, you get the full tourism. (same function with resorts outside 3rd ring... they will generate no tourism)
Now, tiles which are not workable (outside 3 rings of owning city) have a fixed appeal of zero regardless of any factors influencing appeal. And hence, you cannot form parks containing them. I do believe there is a bug when you switch the ownership to another city... the appeal sometimes remain zero even when the new owner city has it within 3rd ring.