I'm sure I've read it somewhere but I can't find it.
Are the bottom two centre terrains, mountain and forest, joined and the hill terrain above not on the edge or is the hill terrain on the edge making the mountain and forest not adjoining?
It's quite a badly designed part of the map, tbh.
The two are touching and the hill is NOT on the edge. It seems quite clear to me. You have a complete border around the Hill region and it is not touching the edge.