First time I have ever had this. Working on a tiny world with islands. Wanted to add another one or two. They will not route in the center as soon as worked on at all after created. Can just smooth edges and interior no longer routes. Any ideas as to what the heck is going on? Have tried about 5 times to do this. The rest of the world is fine.

First pic: routing grid is there, just covered by elevated terrain

3rd pic: this happens on entirely flat, empty areas. As soon as you place lots, roads, decor the grid will be fine again. If you want a visually entirely empty island you can sink small rocks into the ground, this will work for the grid, too

4th pic: see comment 1st pic

Have you tried placing some object on them and rebuilding the routing grid? Especially in combinatin with water routing the grid sometimes behaves very weird...

I usually just put plants or trees, and rebuild routing. On an island which is higher in the middle, you might need something on the higher ground, like a tree or something ... or stones like Norn says.

Thank you for the replies. I will do more work on them. Did not know that was needed. Will try again. I was looking too soon it seems. Guess the game needs something to actually route around. Makes sense.

edit 1-10: am totally back in business. Learned something new. Thank you all!