I am pissed as you are about this issue, but it is unfortunate to acknowledge that these annoying border wall glitches will never be removed, unless I redo the entire map (which... *we* won't).
I made this map manually that's why it popped up, unlike importing the ready-made terrain png which would definitely not create this problem.

So I recommend just imagining that it doesn't exist, or change your render distance so you won't see it.