In the example of the bee texture, you can see the texture is fairly bright, however when its applied to a custom model in Minecraft it appears to darken the texture no matter what item it is.

Previously this happened with just Spawn Eggs because of the overlay, however now it appears to affect all items!

In the other example, an item was made to look like a crafting table UI. The texture itself is bright and in 1.14 it was seamless however in 1.15, no matter how bright you make it, it will be darkened.