Powder snow's faces all have specific cullface parameters specified within. These work fine for neighbouring powder snow blocks as to allow internal visibility, however, the internal code (the model files are not to blame here) only seems to allow it to cull against other powder snow blocks. Despite the model implying otherwise, there is absolutely no culling (at least of the outer faces) against full solid blocks, resulting in faces that will never otherwise be visible being rendered and preventing viewing of powder snow rich areas properly via Spectator due to dark planes.
Compare to water or lava against a wall - the face connected to the wall is culled as expected.
Preferably only the exterior faces should be culled by solid blocks - the interior faces should not be, and should only be culled by other powder snow blocks.
muzikbike
Can confirm