Find my network - delinked phone Passcode

Hey there, I tried to turn on Find My network on my iPhone 13 Pro. It asks for advanced security and then for my Passcode. Afterwards it also asks for my old iPhone's Passcode ("Enter Passcode for Other iPhone"), which has been delinked from my account ~2 years ago. Why? How can I circumvent this?

The "Other iPhone" doesn't show anywhere on iCloud, or on my profile (as I have deleted it from there), so I don't understand how this comes up in their system. The only alternative that the phone gives is deleting all my encrypted data from my iCloud - which I obviously don't want to do.