Glad it worked.
I can't know without seeing it, of course, but I think what was happening is: You weren't restoring the backup you made in MC19, but were restoring one of the automatic backups that MC20 makes itself. If you just opened it up and clicked Save in the first spot MC19 wanted to save the backup, this is NOT the same folder that MC20 would be looking in for the backup.
MC19 uses: <User Directory>\Documents\JRiver\Media Center 19\Library Backups
MC20 uses: <User Directory>\Documents\JRiver\Media Center 20\Library Backups
Notice, the 19 and 20. Those aren't the same folders. They might look the same, because they'd both contain backups in ZIP files that would work, so it might look confusing. I think you made the backup in the MC19's version of the folder. Then opened MC and looked in the MC20's version of the folder for the backup. The one you found was not the same one you just made a minute ago, but one of the automatic ones that MC20 has been making by itself.
My theory anyway. Glad you got it sorted, again.