Moving shader code to helper function doesn't work - Compatibility Mode

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Godot Version

v4.3.stable.official

Question

Hi - i’m following along with the advanced post processing docs -

the following shader works and produces the desired effect:

shader_type spatial;
render_mode unshaded, fog_disabled;

uniform sampler2D depth_texture : hint_depth_texture;

void vertex() {
  POSITION = vec4(VERTEX.xy, 1.0, 1.0);
}

void fragment() {
	float depth = texture(depth_texture, SCREEN_UV).x;
	vec3 ndc = vec3(SCREEN_UV, depth) * 2.0 - 1.0;
	vec4 view = INV_PROJECTION_MATRIX * vec4(ndc, 1.0);
	view.xyz /= view.w;
	float linear_depth = -view.z;

	// Visualize linear depth
	ALBEDO.rgb = vec3(fract(linear_depth));
}

However, when I try to use the same code in this DepthVS function, the shader no longer works. It also doesn’t produce any console errors.

shader_type spatial;
render_mode unshaded, fog_disabled;

uniform sampler2D depth_texture : hint_depth_texture;

void vertex() {
  POSITION = vec4(VERTEX.xy, 1.0, 1.0);
}

float DepthVS(vec2 uv, mat4 inv_projection_mat) {
	float depth = texture(depth_texture, uv).x;
	vec3 ndc = vec3(uv, depth) * 2.0 - 1.0;
	vec4 view = inv_projection_mat * vec4(ndc, 1.0);
	view.xyz /= view.w;
	float linear_depth = -view.z;
	return linear_depth;
}

void fragment() {
	float linear_depth = DepthVS(SCREEN_UV, INV_PROJECTION_MATRIX); // todo: for some reason when separating below code into separate function it doesn't work
	// Visualize linear depth
	ALBEDO.rgb = vec3(fract(linear_depth));
}

This only happens in Compatibility mode. Switching to forward+ (and accounting for clip space changes) works fine.

Any idea what might be going on?