Hi there!

I am currently working on an 3D model an entire street.
However, sometimes when I draw a new face, it disappears after I save my model.
Which is pretty annoying, considering I cannot undo after saving, so I'll have to draw it again.

For example:

This is before saving.

And then after saving this happens (notice the face at the front has disppeared)

And if I Undo, this happens

Can someone explain what is causing this?