(NOTE: currently not able to test this in-game to see if Sodium fixes it, I will do so later)

There exist an assortment of small problems with rendering lighting in vanilla resulting in stuff appearing darker than it should.

For blocks, there's https://bugs.mojang.com/browse/MC-123703 and https://bugs.mojang.com/browse/MC-233461 (possibly a duplicate), which prevent blocks intended to be solid colors such as white from actually achieving that color due to the game erroneously capping it. Both of these tickets include minor code analyses and fixes.

Items in the inventory are also subject to more extreme artificial darkening due to rendering changes in 1.15: https://bugs.mojang.com/browse/MC-213446

And likewise for these pure white blocks (I haven't explicitly disabled AO or shade, however):

Definitely seems to be happening for items, as this is 252 252 252 instead of the raw texture's 255 255 255:

May relate to #883