Blocks that have transparency and use the 'alpha test' rendering method have blacked out insides as well as covered up blocks the player can see through the transparent block.

This has been discovered while using an addon where their custom leaves use "alpha_test". Which made blocks inside these leaves blocks become black.

See attached image for an example.
The left side is the leaves block in question with the inside blacked out.
The right side shows what the inside block looks like (using 'opaque' for its rendering method).

I am not sure if this has to do with any changes to the way the game does backface culling. As this bug started showing up in recent updates.