Render to DataTexture2DArray fails

HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Render to DataTexture2DArray</title>
</head>
<body>

<script id="vs" type="x-shader/x-vertex">
    void main() {
        gl_Position = vec4( position, 1.0 );
    }
</script>

<script id="fs" type="x-shader/x-fragment">

    precision highp sampler2D;
    uniform vec2 texSize;
    uniform sampler2D tex;

    void main() {
        vec2 coordinate = gl_FragCoord.xy / texSize;
        gl_FragColor = texture(tex, coordinate);
    }

</script>

<script id="fs-array" type="x-shader/x-fragment">

    precision highp sampler2DArray;
    uniform vec2 texSize;
    uniform sampler2DArray tex;
    uniform int layer;

    void main() {
        vec2 coordinate = gl_FragCoord.xy / texSize;
        gl_FragColor = texture(tex, vec3( coordinate, layer ));
    }

</script>

<script src="https://threejs.org/build/three.js"></script>

<script type="module">
    import {WEBGL} from 'https://threejs.org/examples/jsm/WebGL.js';
    if ( WEBGL.isWebGL2Available() === false ) {
        document.body.appendChild(WEBGL.getWebGL2ErrorMessage());
    }
</script>


</body>
</html>

JavaScript

const textureWidth = 1;
    const textureHeight = 1;
    const numLayers = 2;

    // Setup scene etc.
    const scene = new THREE.Scene();
    const camera = new THREE.Camera();
    const renderer = new THREE.WebGLRenderer();
    const mesh = new THREE.Mesh(new THREE.PlaneGeometry(2, 2));
    scene.add(mesh);

    // Render
    renderTextureTest();
    renderToTextureArrayTest();

    function renderTextureTest() {

        console.log('Render to DataTexture');

        const originalTexture = createOriginalTexture();

        mesh.material = createShaderMaterial();

        const renderTarget1 = createRenderTarget();
        const renderTarget2 = createRenderTarget();

        renderTextureToTarget(originalTexture, renderTarget1);
        console.log('Result after first render', readOutput(renderTarget1));

        renderTextureToTarget(renderTarget1.texture, renderTarget2);
        console.log('Result after second render', readOutput(renderTarget2));

    }

    function renderToTextureArrayTest() {

        console.log('Render to DataTexture2DArray');

        const originalTexture = createOriginalArrayTexture();

        mesh.material = createShaderArrayMaterial();

        const renderTarget1 = createArrayRenderTarget();
        const renderTarget2 = createArrayRenderTarget();

        for(let layer = 0; layer<numLayers; ++layer) {
            renderTextureToTarget(originalTexture, renderTarget1, layer);
            console.log('Result after first render of layer ' + layer, readOutput(renderTarget1));

            renderTextureToTarget(renderTarget1.texture, renderTarget2, layer);
            console.log('Result after second render of layer ' + layer, readOutput(renderTarget2));
        }
    }

    function createShaderMaterial() {
        return new THREE.ShaderMaterial({
            uniforms: {
                tex: {value: null},
                texSize: {value: [textureWidth, textureHeight]},
            },
            vertexShader:...