<< Before you do that, you may want to grab your boundry edges to use to retrim the larger untrimmed surface afterwards.

From the top view terrain is not a perfect rectangle!
Does this normal ?
So the little parts who must close the "open sides" are some random to choice as the good "boundry edges"?
You closed this "2 boundry edges" missing by hand ?
Maybe some tiedous if you have hundred of these ? (multiple blending don't exist ? Extend don't work ! A closing curves function? )

the surrunding border "grabing edges" before the untrim! :)