That can happen if the face is a long way from the origin. If the corners are not exactly coplanar, but they are near the origin, only one face is drawn. When it’s a long way from the origin the diagonal line that you draw causes two new faces to sit on top of the old one.
Here’s a topic I started in September 2019, talking about the problem. You can see the screen recording I added to one of the posts, to see how drawing a diagonal edge created two overlaying new faces. The extension sWilliams wrote solves the problem.