I'm having a similar issue, but my variant suggests it's not a one-deck limitation but some export "feature" of new iPhones.
I had ten decks or so restored from my iPhone 6 backup to my new iPhone X. All still work fine, but when I tried adding some new decks, I had problems. Two decks exported fine, but every attempt to add others simply replaced the second with the new one.
David, can you help?