Not able to read pixels from offscreen render target in filament web

I am trying to render a sample triangle to a offscreen render target and trying to use readPixels API to get the pixels buffer on the web. First thing i observed was that there was no readPixels API exposed in jsbindings for filament.js (Not sure if this is a bug or not part of roadmap). Then i tried to compile the c++ sample code directly into wasm and render it from there. I was able to successfully render the triangle on screen but doing the same to a offscreen rendertarget fails with no callback coming into PixelBufferDescriptor.

I am using the below code for offscreen rendering and reading:

using namespace filament;
filament::Texture* offscreenColorTexture = Texture::Builder()
.width(256)
.height(256)
.levels(1)
.usage(Texture::Usage::COLOR_ATTACHMENT | Texture::Usage::SAMPLEABLE)
.format(Texture::InternalFormat::RGBA8)
.build(*app.mEngine);

filament::Texture* offscreenDepthTexture = Texture::Builder()
.width(256)
.height(256)
.levels(1)
.usage(Texture::Usage::DEPTH_ATTACHMENT)
.format(Texture::InternalFormat::DEPTH24)
.build(*app.mEngine);

app.mOffscreenRenderTarget = RenderTarget::Builder()
.texture(RenderTarget::AttachmentPoint::COLOR, offscreenColorTexture)
.texture(RenderTarget::AttachmentPoint::DEPTH, offscreenDepthTexture)
.build(*app.mEngine);

const size_t byteCount = 256 * 256 * 4;
uint8_t * buf = new uint8_t[byteCount];
memset(buf, 0, byteCount);

filament::backend::PixelBufferDescriptor buffer(
buf, byteCount,
filament::backend::PixelBufferDescriptor::PixelDataFormat::RGBA,
filament::backend::PixelBufferDescriptor::PixelDataType::UBYTE,
[](void* buffer, size_t size, void* user) {
// print data here (callback not getting executed)
uint8_t * buf = (uint8_t *)buffer;
for(int i=0; isetRenderTarget(app.mOffscreenRenderTarget);
app.theView->setPostProcessingEnabled(false);
app.theRenderer->renderStandaloneView(app.theView);
app.theRenderer->readPixels(app.mOffscreenRenderTarget,0, 0, 256, 256, std::move(buffer));
app.theRenderer->getEngine()->execute();
app.mEngine->flush();

**Desktop (please complete the following information):**
- OS: web
- Backend: [OpenGL]

**Additional context**
Threading is disabled for filament on web. Hence i am calling flush API of engine and not flushAndWait API.