optimized media and project exports

I am working in Resolve Studio 15.0.08.057
PC based. I have created optimized media for my
project. The optimized media is working fine in my project.
I exported a backup of the project and
imported this backup back into Resolve. The optimized media is gone.
"rediscover optimized media" does not work.
When I "save project as" the new project retains the optimized media.

Any ideas?
Is this a known bug with exported projects?
Thanks,
CG