Ambient light acts completely random

I have a few meshes that essentially are just fancy planes. I'm planning to use these as repeatable tiles to construct my world from.

However, I'm getting strange lighting issues (the animation below is with all static meshes and after waiting for the lighting build to complete):



The issue appears to be that ambient lighting is completely ignored on some of the tiles. Here's what happens when I turn ambient lighting up and down:



My best guess is that Unity is doing some interior/exterior determination to decide whether a mesh is influenced by ambient light and that this logic is somehow failing for my shapes.

Can anyone explain this or better yet, provide a workaround?

 

Maybe worth noting, my meshes have invisible "blinds" that cover places where light normally doesn't reach (eg. inside the cutaway view of the ground). Their mesh renderers are set to "Shadow Only"

If I made them visible, they'd look like this (the upper quad stops right where it hits the grass):

 

• Created a new Unity project
• Imported just the vanilla meshes using default shader
• Placed the meshes in a scene and set them to 'static'

As soon as lighting bake finishes, two of them become darker and receive black shadows. Disabling lighting and/or reflection probes also doesn't change anything.

Since I'm not doing anything fancy in this scene, I consider it a bug and reported it as such: Issue #696594