Create a box, any size. Make the top box full cobblestone, there is 0 light, the dreadful dirt works perfectly in said environment.

Now, replace top block in box with bottom cooblestone slabs, and the sky AND block light level is still calculated as 0 by Minecraft, as it should be, yet, dreadful dirt will burn and disappear, as if it is directly in the view of the sky.

On mob_grinding_utils-1.16.5-0.4.21.jar, forge 1.16.5.36.1.2, playing with ATM6S-1.0.9.

Hello,
IIRC some blocks like slabs and stairs calculate light levels different and as such won't block the light.
for example Zombies will still burn under half slabs that are exposed to the sky.
I'd suggest just using a full block - I mean we have added tinted glass that blocks out light and allows you to see in to mob farms (has connected textures too). If things spawning on the roof are an issue you could always add a layer of half slabs over a full block of cobble etc.
I'm not sure much can be done about this just atm, but I'll put it on my TODO list of things to look at,
Cheers,
Me.

The check for burning/spawning is a combination of a light level and if the block can see the sky. I'm not doing anything special or wonky in the code and am using vanilla methods to check such things. Irrellevant of what the F3 console shows you for the light level, slabs and some blocks may/may not trigger the vanilla method for 'seeing the sky' - I can probably look in to this as I have said, but it is a minor thing for which vanilla mechanics are really what are being quizzed tbh. :)

*Edit
The code for the block to catch fire is based on what time it is in world (eg day to night) and if the block can see the sky
Light levels only effect the spawns. So it may be that slabs don't tell the block under that they are blocking the sky.