Sodium fixes some aspects of how block models are shaded, which resolves a vanilla issue which causes blocks such as farmland and dirt paths to appear dramatically darker when next to opaque solid blocks (#50). However, there are still some residual issues with incorrect model shading which Sodium does not resolve, which are noticeable on vanilla blocks such as slime blocks and mangrove roots.

This issue has been reported under https://bugs.mojang.com/browse/MC-261602. A resource pack is attached to that ticket which can also be used for testing Sodium with the same reproduction steps and instructions.

Compared to vanilla, the effect is less extreme in Sodium when using smooth lighting, however not by too much. Here's a comparison at 50% brightness, smooth and non-smooth lighting:

The same at moody brightness:

It'd be expected that the two planes of the two blocks in question which are coplanar would be shaded the exact same, since they're at an identical distance from the solid block they're against. However, the plane belonging to the glass block which is touching the stone block is far darker than the one not touching the stone block.

Please specify that the screenshots are comparing Sodium with itself, not vanilla. I was confused by this at first.