Maybe a mip issue? Try setting the UI texture group for your texture may help, or force nomipmaps. In the material you can force mip level 0, in your Param2d node change mipvaluemode to mipvalue, then plug a 0 constant into the level that shows up on the node.
So the solution is to set both the Texture2DArray and the Texture to MipMaps Blur1.
Then in the material, control the resolution dynamically using the Mip Levels