This is normal, try adding more edge loops to your mesh - polycounts were often a little higher than you’d expect to accommodate this.
If you want to disable it completely, comment out this line: https://github.com/MenacingMecha/godot-psx-style-demo/blob/91bd24988f70636efca505e2c8022b143b61780e/shaders/psx_base.gdshaderinc#L55