Insufficient info.
No blend file supplied to review! Why Not ?
Are the faces planar
Have you any internal faces where there shouldn’t be
Do you have duplicate vertices where you shouldn’t be
Are the face normal directions all consistent
Any custom normal data in your file ? If so have you cleared it in the Mesh Geometry Data properties
Any modifiers ?
“Are the faces planar”
Yes
“Have you any internal faces where there shouldn’t be”
No
“Do you have duplicate vertices where you shouldn’t be”
No
“Are the face normal directions all consistent”
Should be
“Any custom normal data in your file ? If so have you cleared it in the Mesh Geometry Data properties”
No custom normals, just a raw imported OBJ.
“Any modifiers ?”
No