There it is. The edges match now.
I just remembered that a tileable bitmap doesn’t have the leftmost column of pixels equal to the rightmost one. Not the top and bottom lines do. Which works for images but for not terrains doesn’t because a terrain system needs to create the mesh to even those columns and lines of pixels.
So I altered my bitmap to have the exact same line of pixels on the top and bottom. And same for left and right most columns. And now it matches.
That was a hiccup on my brain that I didn’t figure this out earlier.