I found that the light level calculation doesn't work as intended in 1.0.20.
Here is what my mob farm looks like: it was a box built with tinted glass and stone, and a door using Mobfilter from dark utilites.

However at the far corner of the box the light is larger than 7, but it should be 0.

Here is a world which also faces the problem.
testworld.zip
I re-enter the world many times and found that this issue may appeared randomly. For some reason I don't know it have been repaired.

Here is the same.
My mobfarm is completly disabled and I have no idea to get rid of the light.

I have same issue

I also had this problem after upgrading from 1.0.19b to both 20 and 21b.
My mob farm was based on Dreadful Dirt, and the dirt burned (exposed to daylight) after I broke the Vector Plate (dark utilities) that was sitting on it, even though I know it was under a roof.
Edit: "Had" this problem because I rolled back to 19b due to this and a Create issue.

Same issue here using 1.0.21b. Have not found a workaround.

Any idea if it's tied to upgrading from a previous version? Or does it work properly if you create a new cursed earth mob farm after you're running 1.0.21b already?

@MrChunks No it doesnt have to do with upgrading, I made this world on 1.0.21b from the start. My farm intersects the chunk boundary, going to see if making a new farm not on chunk boundary could fix it.

Gotcha. I was also wondering if it had anything to do with the special tinted glass. I shoulda tested more before I rolled back.

im also having this issue, i tried to use dreadful dirt as a substitute instead however it seems that solution was only temporary as its now completely stopped i assume the dirt has now been burned (i havnt checked because the mob spawner is surrounded by a aquarium

Based on my testing, I believe this issue is at least in part related to Starlight (unsurprisingly given that it's optimizing the lighting engine.)

On the prior version of the pack, I was experiencing the dark spot bug with Radon, so I swapped it out for Starlight and instantly began having a bright mob farm.

That has continued in this version of the pack. Sometimes I've been able to solve this by filling every block inside my farm with dirt/cobble and then vein mining it out, but not always or not always completely.

I've since removed Starlight and so far I haven't encountered the issue.

Edit: I should note that I haven't really experienced any noticeable performance degradation after removing Starlight. However, I have a reasonably strong PC.