When switching my server from Starlight to Phosphor it seems that the light rewriting is having an issue where a given chunk is not checking for light being produced from sources in surrounding chunks, or maybe the opposite. Since Phosphor uses the vanilla format for saving light I assume this also happens switching back to vanilla but thats not tested by me.

You can see its not a rendering thing with f3 showing light 0.

Does this happen switching to Vanilla?

No response