Help getting rid of random shadows

Okay, I found the issue after tearing this mesh apart one more time. It's the bottom plane of those planks/slabs. Highlighted in the image here: imgur.com/gallery/tMngOCq

This does look kinda weird in the game without them. My question now is if there is a way to add a bottom portion without getting that shadow back because then I cannot make the arch too tall.

This mesh doesn't have a shadow layer on the mesh itself but it has a shadow specific mesh only.