I'm the creator of the John Smith Legacy Models Addon. When using Sodium, certain parts of models from my resourcepack appear dark, namely the "better grass" and the glowstone and sea lantern. For the latter two, I thought initially that it's the optifine style emissivity texture, but after removing that it didn't fix anything. I attach some screenshots.

The specific screenshots are using the alpha 1.18 builds, but the same issue occurred in previous versions