Thanks for the replies, Patience and Hiker, I think I found the problem.
The incremental saving didn’t help much in this case. I had 10 versions of the model. When the problem occurred in the 10th version. it also started occurring from version 5 on up.
I exported the model as an .obj and looked at it in Carrara and saw the same thing - many pieces missing. I then pulled it into UnWrap3d where all parts were visible, and found issues with some boolean operations. I couldn’t fix those issues in UnWrap3d, so I exported the model as another .obj and imported that into Carrara for repair. Once done, I exported that as a third .obj and was able to view and work with it inside Hexagon.
Another thing fixed in UnWrap3d that may have contributed to the problem was very long group names resulting from nested groups in Hexagon. I shortened these up in UnWrap3d before I was able to see the complete model in Carrara. If there’s a character limit on group names, Hexagon could have gotten confused on multiple groups having the same truncated names.
If I had to guess though, my money would be on the booleans as the culprit.