So the issue here is that you have a Terrain Warp operator at the end of your operator list, which is set to 'Clamp' at the edges...so when the warp function is sampling near the edges it's clamping the values that go outside, so you're seeing those clamping artifacts.
Normally you'd just switch to 'loop' mode, but that only works if the edges of your terrain are all roughly the same height...in this case, they're not...because your terrain has a slight gradient.
A workaround could be to add a Terrain Tile operator before your Warp operator and set the mode to 'Loop terrain edges'. That will make the terrain tileable by ensuring edges in all directions match the opposite side of the terrain. Then after you can put your Warp operator and switch its mode to 'loop' too.
Attached is a file showing how this is done.
Normally you'd just switch to 'loop' mode, but that only works if the edges of your terrain are all roughly the same height...in this case, they're not...because your terrain has a slight gradient.
A workaround could be to add a Terrain Tile operator before your Warp operator and set the mode to 'Loop terrain edges'. That will make the terrain tileable by ensuring edges in all directions match the opposite side of the terrain. Then after you can put your Warp operator and switch its mode to 'loop' too.
Attached is a file showing how this is done.