I am in a completely encased area with mob grinding utils tinted glass therefore no light should get through. However, there are invisible light sources I can't get rid of which means I can not make a mob grinder.

This is how I got around this in my world:

• Disable starlight mod
• Open the pack
• Click on single player
• "Select" your world
• Click edit (i think? not at home)
• Click optimize world
• Check the box for "Erase cached data" & make backup
• Wait for it to finish, it will take a hot minute
• Load your world and check light levels
• You should recheck all your "safe" areas as there may be changes to light levels there

Yes optimize would is a thing: https://minecraft.fandom.com/wiki/Optimize_world

Found this workaround here: https://old.reddit.com/r/allthemods/comments/15liybc/i_found_a_fix_for_the_light_engine_bug_in_atm8/

If I didn't disable starlight mod and used a light source in the mob spawner (to turn it off etc), it would become permanent even after busting the cache w/ the optimize world. It seems starlight caches light sources heavily w/o really checking if they actually exist later (that's my guess anyway).