I've got a strange bug in Maya 2018: On some meshes, after some playing with them - combining, editing, changing vertex normals - normals displayed in the viewport break. This is only an issue of the viewport, after opening the scene with 2017 or exporting meshes everything is ok. This issue only occurs on Maya shader, like Lambert or Blin. E.g. Stingray looks ok.
Maya 2018 - Lambert
Maya 2018 - Stingray
Maya 2017 - Lambert:
I've reported this bug to the Autodesk, but I'm still looking for some workaround. Switching viewport renderer does not change anything.