Artifacts/"border" around HighPassTexture

I have a rendertarget, looking like this


I want to use it as TextureObject and run it through a HighPassTexture with a contrast of 48, but I’m getting some results I can’t explain:


It became very apparent when I changed the render target resolution to 1024 from 256, but it did exist before that. This obviously creates problems when used as a tiling texture on terrain.

I’m quite new to using unreal and creating materials, and have no idea what to do from here.