OpenGL/WebGL - Incorrect depth when rendering to texture/framebuffer twice?

I am trying to implement volume rendering on a simple model with webgl. For this to work I need the backfaces in a seperate texture, simply by rendering a simple pass to texture like this:

var tx = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, tx);
gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.drawingBufferWidth, gl.drawingBufferHeight, 0, gl.RGBA, gl.FLOAT, null);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
var fb = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, fb);
gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, tx, 0);
//setup viewport
gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);
gl.clearColor(0.9, 0.9, 0.9, 1.0);
//setup culling and depth testing
gl.enable(gl.CULL_FACE);
gl.cullFace(gl.FRONT);
gl.enable(gl.DEPTH_TEST);
gl.depthFunc(gl.LESS);
//render elements
if (model.index) {
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
    gl.drawElements(gl.TRIANGLES, numItems, gl.UNSIGNED_SHORT, 0);
}
else {
    gl.drawArrays(gl.TRIANGLES, numItems, 0);
}

this works like expected, the fragment shader simply is a:

gl_FragColor = vec4((0.5 * v_pos) + 1.0, 1.0);

However when I then try to pass this texture from the framebuffer into a new shader where i want to perform the actual raymarching/volumerendering like this:

//2nd renderpass to canvas
//switch program
//set current shader
context.shader = volPrgr;
//activate the shader
context.gl.useProgram(volPrgr);
var gl = context.gl;
var shader = context.shader;
if (!shader) {
    return;
}
init(gl);
gl.bindFramebuffer(gl.FRAMEBUFFER, null);
//setup viewport
gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);

//init buffers
.
.
.

gl.activeTexture(gl.TEXTURE0);
gl.bindTexture(gl.TEXTURE_2D, tx);
//gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.drawingBufferWidth, gl.drawingBufferHeight, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
var textureLoc = gl.getUniformLocation(context.shader, 'u_back');
if (isValidUniformLocation(textureLoc) ){
    gl.uniform1i(textureLoc, 0);
}else{
    console.log(textureLoc + " is not a valid texturelocation.");
}
//send screensize needed for texture lookup
gl.uniform1i(gl.getUniformLocation(context.shader, 'u_screenRexX'), gl.drawingBufferWidth);
gl.uniform1i(gl.getUniformLocation(context.shader, 'u_screenRexY'), gl.drawingBufferHeight);
gl.disable(gl.CULL_FACE);
gl.enable(gl.DEPTH_TEST);
gl.depthFunc(gl.LESS);
//setup viewport
gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);
gl.clearColor(0.9, 0.9, 0.9, 1.0);
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
//render elements
if (model.index) {
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
    gl.drawElements(gl.TRIANGLES, numItems, gl.UNSIGNED_SHORT, 0);
}
else {
    gl.drawArrays(gl.TRIANGLES, numItems, 0);
}

and in the shader:

vec2 lookup = (vec2(gl_FragCoord.x/float(u_screenRexX), gl_FragCoord.y/float(u_screenRexY)));
vec3 back = vec3(texture2D(u_back, lookup)).rgb;
gl_FragColor = vec4(back.rgb, 1.0);

I get this when simply trying to render the texture onto the model using screenspace coordinates:

How is this possible? Can I change the Depth ordering for the volume shader for it to dsiplay the model/texture correctly (head of the bird in front of the wing)? Thank you