FloodFill is working with offhand light, in example lantern, but doesn't work with trinkets slot's, in my example lantern on belt. I'm using BackslotAddon mod based on trinkets for it.

It would be cool and suuper handy if lantern on the belt will emitt light as same as a lantern in offhand!

the shader can only detect if the light is in your main hand or your offhand. if it is in some third place added by a mod, it is not a situation the shader is aware of

https://legacy.curseforge.com/minecraft/mc-mods/immersive-lanterns
Use this mod to make accessories slot considered offhand for lighting