Both versions of Patcher (Sk1er and Poly) have this lighting engine bug.

[WhereIsDaLove]

So I've come across this lighting bug that happens when using either version of Patcher for Minecraft.
Basically, the lighting does not update when any block is exposed to the sky.

You can replicate this by creating a new world, digging a hole in the ground, and then making a small tunnel. Next, close the hole, and you will see that the lighting doesn't go dark. It only seems to happen when the hole is exposed to the sky.
I double checked and also made sure to see if maybe there was some mod conflict happening, as well as going through settings to see if maybe a Fullbright feature was turned on by mistake, but to no avail.
I even ensured that only Patcher was installed, and it still happened. (I tested it with both Sk1er's and PolyFrost's versions.)

This image is what the tunnel looks like before opening a hole.

Here I made a hole to expose the tunnel to sunlight.

Then I covered it up, and you can see how the lighting doesn't update.

Even covering it up further doesn't make it dark.

[WhereIsDaLove]

I do wanna make this clear, I made sure that Optifine wasn't installed when this happened, as I know that it can be a cause for some issues here and there.