Grades from Manually Created Grade Items Missing After Restore

I'm trying to create new courses using existing ones on Moodle 3.5.1 but even though I backup and include grade history, once I restore and go to the gradebook settings, it recalculates and removes the categories and grade items. Grades associated with activities remain. I researched this forum and found previous posts in this regard with no solution. So, if you want to back up a course and download it as an archive, later when you restore it because you need some past grades, they'll be missing so what am I doing wrong? How can we keep all the categories, grade items, and student grades after restoring a backed up course?

Thanks for any help.

David