I assume you’re restoring via the GUI, you may get different result if you restore using the CLI:
fwconsole backup --restore /path/to/restore_tarball.tgz --restorelegacycdr
Also you can manually copy CDR records between major versions using the method in part 2 of this post: How to: Upgrade FreePBX13 to FreePBX14 on Hyper V - Guide