When using the shader on Iris, the reflections of the sun and the moon are the shader ones and not the vanilla/resource pack ones:

To reproduce :
Use shader on iris (or optifine).
Set Sun/Moon type in Atmospherics to Vanilla.
Use PBR or SSR reflections in any way, looking at the sun/moon.

I don't know if it's a resource pack incompatibility or a vanilla one. Will need a bit of testing. Anyway your shader is GREAT!

Hello, thank you for the compliment. Unfortunately, I currently have no way to actually reflect the sun and moon from a resource pack or even the vanilla sun and moon so only specular reflection is used.