I have this strange shadow behave on render in Cycles. Simple one side floor is rendered correctly if Z location is > 3cm. There isn't any other geometry and no overlaping with walls. I'm new in Blender and I newer saw something like this in other 3d software/ rendering system. Thank you for the explanation.
First of all, it was looking like Z-fighting but it is not. I've checked twice normals, doubles, holes and everything else what comes to my mind, nothing was an issue but...
Vertices are not on the same Z location (I don't know which one) but after selecting all floor vertices - A then S > Z > 0 (it will place all of them on Z=0) everything is ok.
Little tips: this machine is hovering over the floor and use Filmic Blender for this ;)