When one or more faces of a mesh node disappear, instead of modifying the whole model in the original 3D modeling tool, you may use the Flip Face Mode feature provided by 3DXchange to fix the issue.
Face break
Press the 2-Side Picking tool in the Tool Bar.
Click the Flip Face Mode button to toggle the mode.
Directly click on the missing face or faces in the Preview Window.
Click the Flip Face Mode button again to leave the mode.