Using the latest release of 1.16.5, then teleporting using the teleportation crystal for spawn and the teleporter for a custom location (64 use). The client light will become zero for all light sources from blocks modded or not. Use for a second time fixes the light. This also means that mobs spawn at those locations.

Relogging fixes it the first time but the second use turns all signs, chests, and heads back, after 2mins idle, it turns several more blocks turn dark like beds the book for enchantment table, a single chunk, and the client light is still zero.

The server side gives a light value but mobs still spawned even right next to a light source like a sea lanterns and lamps

The same thing goes for the teleportation star

This just sounds like a client sided issue because light levels shouldnt change from an item (unless you have dynamic lighting). Does the issue go away when you save and quit then rejoin the world