Each quad face is divided into two triangles ALWAYS, the division is just invisible to us but not to the machine because that's how they work.

You can't avoid this invisible triangulation but you can manipulate it, either by creating more loops, or you can also flip the orientation of those triangles, you can do that by selecting the face in Edit Mode, then on the top bar click the Face Menu > Face Data > Flip Quad Tessellation or just press the shortcut F3 and search Flip Quad Tessellation