Hey there. So the way the shader works, there's not much I can do to address this in the short term. RenPy sizes the text area to be at its largest even if it only draws text into a smaller area. The shader works by filling and scattering in the drawable space, and when that drawable space is larger than the actual draw, these sorts of things will happen.
The fastest and probably least fun way to fix this would be to edit the font to remove the extra space.