iCloud backup from new iphone is lost when restoring old backup
Apple has no merge feature in any iOS backup. The restore operation implies a blank slate, tabula rasa, wiped device for iOS and TVOS and iPadOS. 

The solution is to store data in a cloud synced manner or otherwise import data but backup is purely for a clean restore. 

Worst case you restore a backup and export or sync the data you need so you can wipe and restore and then Merge manually on the more current backup. Then layer on the loud synced data or imported files.