Blocks in the deferred rendering preview with 2D block faces that have a light point on them only render one face of the texture.
anything that has 2D textures that can use a light point will do this, amethyst, Redstone torch, glow lichen, Fire, only one side of the texture is visible at once.

Reproduce:
make a resource pack that gives a light point to any block that gives off light that has a 2D texture in it
or import the attached pack and place amethyst buds in a world set up for deferred rendering preview.

Expected: 2D block faces should render on both sides regardless of where the player is.