Transparency does not seem to properly be applied in your example. You find a slightly improvement by adding in setup:
hint(DISABLE_DEPTH_TEST);
hint(ENABLE_DEPTH_SORT);
Now you can see the top sphere shows transparency but the bottom sphere does not have any. I do not have an immediate solution but I would like to record the next two post for reference:
- Previously similar issue: Draw-order based transparency issue
- Draw order in 3D
Kf