Hi Danny,

>>then why does it have a surface missing

Because it is an invalid surface...

When you explode that solid, hide all surfaces except that invalid one (represented by its edges only), then push Sel all or Ctrl+A (only way how to select that surface) and pick one edge and push Sel all or drag a selection window to select all edges. Now, Copy those edges to clipboard and hit Delete and Paste to back an underlying untrimmed surface and trim boundary respectively. As you can see, the underlying surface has nothing to do with outer trim boundary of your missing surface.

Petr