The geometry looks as I intended it to look in Solid mode, everything correctly, like this:
But upon entering mat. preview or rendered mode it looks flat shaded, as the mesh is, also for some reason looking reflective as if roughness was at 0 (which is the whole opposite) at least that is only in mat. preview
I already tried some solutions of similar problems here, like unchecking auto smooth, clearing custom split normals data, joining by distance (deleting vertex doubles), etc..., but none of them worked, apart, some even messed up how it looked in solid.
It doesn't have any modifiers.