Long story short, the problem went away after installing an old system image and I also dumped all the original clips files and reloaded them from backup copies. The first time I reloaded the project, Studio re-rendered as had been the problem.  But upon subsequent re-loads, the project does not do a full re-render. And... with the new OS image, optimization is again at default = 100, so that isn't/wasn't the issue.  I wonder if the render folder was corrupt or self-deleting; albeit I don't know when that folder is used/accessed?

 If the problem recurs, I will try Traveller's suggestion.  That sounds like a valid avenue.  If that works, it certainly sounds easier than replacing the OS - as well as it would confirm that my solution wasn't necessary.

Thanks to everyone  for your help!