From https://atadistance.net/2018/04/13/jr-east-and-apple-quietly-fix-a-suica-weakness/:
One of neat things about Apple Pay Suica is how easy it is to add, delete and migrate Suica cards to different iPhones. iCloud and Mobile Suica keep the Suica card information and balance safe and secure no matter what device it's on.
There was one weakness however: the Suica card ID number changed every time the card was deleted and re-installed in Wallet. …
Fortunately Apple and JR East have fixed this weakness. Now when you delete a Suica card from Wallet and re-install it the Suica ID number does not change.
I was annoyed by this "weakness", which probably was a design flaw. Glad to see it is addressed but not looking forward to the next time this applies to me.