Flow map moves slightly up and left when it should stay stationary?

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:bust_in_silhouette: Asked By joeyq

I asked this question on godot discord but it’s also helpful to ask here.
I’m new to flow maps following tutorials like

Flowmaps, gradient maps, gas giants.
Martin Donald

While testing a simple flow map i come to these problems:
1.) the greenish part of the flow map with color (128,128,0) should not move but it slightly goes to the left and up. Why does this happen and how to prevent this?
2.) In Gimp I made the flow map simply with rectangle tool and bucket fill with red. Why is there artifacts between the red and green colors?

shader_type spatial;

uniform sampler2D main_texture;
uniform sampler2D flowmap;

uniform float flow_speed = 0.1;
uniform float flow_intensity = 0.1;

void fragment() {

    // Flow map
    vec2 flow = texture(flowmap, UV).xy;
    flow = (flow - 0.5) * 2.0; // remap to -1..1
    float nice_time = fract(TIME * flow_speed); // time 0..1
    vec2 nice_uv = UV + (flow * nice_time * flow_intensity);
    vec3 main_tex = texture(main_texture, nice_uv).xyz;
    ALBEDO = main_tex;

flow map looks like this, green part (128,128,0) should stay still, red part (128,0,0) should flow downward:
flow map