Oh my, you're right, this solved it for me too! Thank you!
This is my simple shader now:
shader_type canvas_item;
uniform float lod: hint_range(0.0, 5.0) = 0.0;
uniform sampler2D SCREEN_TEXTURE: hint_screen_texture, filter_linear_mipmap;
void fragment() {
vec4 color = texture(SCREEN_TEXTURE, SCREEN_UV, lod);
COLOR = vec4(color.rgb, 1.0);
}
