Why are the frames of my particle so stretched?

The following code allows the texture of a particle be displayed in the form of animation, as well as flip it horizontally if flip == true. However, the frames of the particle are stretched horizontally. What am I doing wrong?

shader_type canvas_item;

uniform int total_frames = 4;  // Total frames in the sprite sheet (4 horizontal frames)
uniform float animation_speed = 1.0;  // Speed of the animation (higher value = faster animation)
uniform bool flip = false;  // Uniform to control horizontal flip of the texture
uniform vec2 sprite_size;  // Sprite size in texture space

void fragment() {
    // Get the texture coordinates (UV) passed to the shader
    vec2 uv = UV;

// Calculate the width of each frame (each frame takes up 1/4th of the texture width)
float frame_width = 1.0 / float(total_frames);

// Calculate the current frame based on time and animation speed
float time = TIME * animation_speed;  // TIME is the built-in variable that provides the elapsed time in seconds
int current_frame = int(mod(time, float(total_frames)));  // Modulo to cycle through frames

// Calculate the X offset for the current frame
float frame_x_offset = float(current_frame) * frame_width;

// Adjust the UV.x to point to the correct frame in the texture
uv.x = frame_x_offset + uv.x * frame_width;

// If flip is enabled, invert the UV.x to flip the texture horizontally
if (flip) {
    uv.x = 1.0 - uv.x;
}

// UV.y based on sprite height to prevent stretching
uv.y = uv.y * sprite_size.y;  // Ensure UV.y stays within the correct height range

// Sample the texture using the modified UV coordinates
vec4 tex_color = texture(TEXTURE, uv);

// Set the final color of the fragment (particle or sprite)
COLOR = tex_color;
}