When I summon a fallingSand entity for stained glass, the block is completely opaque, not at all transparent.

I would normally check that up to the fact that transparent blocks don't render correctly as entities. However, every single other transparent block in the game renders perfectly. Every single transparent block, that is, except for stained glass. Even fences and regular glass work fine.

This is the command I used: /summon FallingSand ~ ~1 ~