The flashing happens because CesiumJS will display a white image until the image is loaded. The extruded height might just be causing a small delay. You could try preloading the image and passing in an image element, instead of a URL, like suggested here: Load image before show by imageMaterialProperty.
I think you’d need to do this with the Entity API, outside of the CZML, since you wouldn’t be able to embed a preloaded image in CZML.