Hi,
Even if "Material > Base > Image Transparent" is checked the exported file doesn't have the parameter "transparent": true in the "material" section (referred to the .json file). Adding this "transparent": true, everything work fine
Refer to this model (goal objects): https://clara.io/editor/cc689cb0-c7f7-4fcb-919b-fae12ca02d23
Link model w/o transparency
Link model w transparency