What's Causing the Lighting Issue In the First Pic?

[Burkha]

I included the mods I'm running on this save. What's up with this?
Edit: Forth image is underneath (without blocks from earlier images)

[Burkha]

[Krock]

Stairs do not block light. Otherwise they would appear black, or stacking them vertically would result in wrong looking dark ground. Use solid nodes to block light.

[Burkhaalt]

Stairs do not block light. Otherwise they would appear black, or stacking them vertically would result in wrong looking dark ground. Use solid nodes to block light.

Interesting. Why do stairs not block light? Is it because of their geometry or are they defined differently than other nodes?

Edit: It's also interesting because I placed a solid node on top of that specific area. The light must be coming from the adjacent stairs

[Blockhead]

Interesting. Why do stairs not block light? Is it because of their geometry or are they defined differently than other nodes?

Minetest's lighting model is simplistic. Either a node passes light, reduces it slightly, or blocks it entirely. There's no allowance for geometric and rotational based lighting. Thus, stairs mod, moreblocks mod and a lot of other mods define all their shaped blocks as light-passing rather than light-blocking.

Imagine a house made out of slabs. If you laid them in normal rotation so all the walls had big gaps you would expect light to pass through; if you laid them upright against each other you would expect light to not pass through. Expectation unfortunately clashes with the reality that it's kind of impractical to correctly calculate lighting. Anyway, passing light by default rather than blocking it at all times seems to satisfy more lighting scenarios.

Edit: It's also interesting because I placed a solid node on top of that specific area. The light must be coming from the adjacent stairs

Sounds about right. Light travels horizontally somewhat; for instance if you had an otherwise unlit underground chamber with a skylight, a patch around the skylight will be lit, not just the one node directly underneath it.