Update -
I've received a further reply from Tech Support:
"I received an update from our development team regarding this issue. We have determined the problem is not due to corruption in your .FTM database file, but is actually a bug in the software. We hope to have this issue resolved in future updates of the software."

I hope everyone who has experienced the issue will also submit a bug report and refuse to accept the inevitable first reply from Tech Support, telling you that your file is corrupted, etc. It took several escalation requests to get past the front line, which is not trained to handle problems.