Banners are rendered too bright in the inventory

This is intentional.

Source? You need a source for WAI

I also think that this is intentional, they only changed the inventory model file to use "front" light instead of "side", so there is a limitation of inventory lighting, that was added in 1.15
I don't think Mojang will create a separate group just for banners, or break compatibility with resource packs. And because that I think it's WAI (or a really long unresolved bug).

Even if intentional it's a bad change, the new rendering looks horrible and if this was marked as WAI then it'll just be suggested to be reverted on the feedback site.

@unknown, I removed 1.16.1 from the list of affected versions as it is caused by the fix of MC-167045 in 1.16.2 Pre-release 1.

Understood, I think I accidentally added that version :|

21w07a.

It should be fixed, when it renders a little bit darker like-block shadow render.

21w11a is marked as affected, however, this appears fixed in this snapshot. I've attached a screenshot. Here you can see that they are quite darker.

@unknown is this still an issue for you in 21w11a?

Appears to have been fixed at some point. No longer an issue in 1.17-rc1.