I have run into this problem multiple times. I build a model and then I save. After it loads to save some faces disappear. I am wondering if anyone else have ever ran into this problem. One way that I avoid this is by grouping the face that disappears and also locking it. Then when I save, it does not go away.
I am curious to see if anyone else has had this happen multiple times as well?
I had it happen in SU 2016 first and had it happen again this morning in SU 2017