Issue Description: Dark square spots remain on Dynmap after a fullrender, for both surface and flat maps. The chunk data appears to be there, so it's more of a lighting issue. Any help here would be much appreciated.

• Dynmap Version: Dynmap-3.1-beta-3-spigot
• Server Version: Spigot 1.16.2
• Pastebin of Configuration.txt: https://pastebin.com/zi5E1nWu
• Server Host (if applicable): Pebblehost
• Other Relevant Data/Screenshots: https://imgur.com/At174hM
• Steps to Replicate: Run a fullrender of the world, these spots are left over after.

[x] I have looked at all other issues and this is not a duplicate
(An issue similar to this got closed for a previous build.)
[x] I have been able to replicate this

See #3565 - this is same issue with same fixes as there (it's not 1.18 specific)

Dynmap doesn't modify world data it just reads the lighting data already in the world, do you see shadows like that in the map?

No, if I go back to where the lighting issues are then it looks perfectly normal.

So is this could be an issue with the lighting data on the map already?

I have the same issue.

@MaximMaximS Did you also do some kind of pregeneration for chunks? I did that with WorldBorder before then generating the Dynmap, which I think is partly the cause of the strange shadows.

I haven't found a solution to this yet, other than manually using CleanLight to update the lighting on the affected chunks.
I have noticed that new lighting issues continue to appear though...

Yes, I used WB Fill.

I think that's the cause then, I used other chunk pregenerators for the rest of my map (FastChunkPregenerator) and there were no lighting issues after that, I'd recommend switching if you haven't already; it's faster too.

it is not the pre-generation
with older dynmap versions these problems did not exist (also with pre-generation worlds)
sometimes there is indeed a shadow in the map but sometimes not
i can confirm this issue