You called the field "creative_group" in the example files, when it should be just called "group". That's also what the Block Wizard exports.
I reckon this is intended since the items would be contained in the shulker box drop itself, which doesn't drop.
We have discovered that the issue affects not only the opacity channel, but also the emissive channel.
The pixels in the attached image are marked with their respective value on the green channel. They have very low emissive values, but render with a high contrast. Which makes it difficult to produce smooth transitions between emissive and non emissive parts of a texture.
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