Any Copycat block that emits light from their block source (i.e if they have Glowstone in it), they properly produce light as expected, however leaving the world (singleplayer) and loading back into it causes the lighting to glitch out and light levels to drop again.
Examples below.
Before relog, using a Haunted Bell to show light levels (presumably shows them as they are server-side)

After relog, Haunted Bell confirms that light has disappeared.

Happens without shaders too.

Using latest version, Forge 1.20.1, v1.2.6
I am using many other mods, including Embeddium and Oculus for shaders.

This is a bug in Create itself unfortunately. Creators-of-Create/Create#4889