Confirm iPhone Passcode to continue using iCloud Message

You have to do it more than once because the vulnerability is present more than once.

Apple changed the way that worked because of a recently discovered vulnerability that would allow a hacker to create an unprotected backup of an iOS device without the knowledge of the owner, so Apple add a requirement to always require a passcode to be entered on the phone before backing up to a computer. This to assure that the backup was intentional.

If you want details you can find them in these two links:

• From the discoverer of the vulnerability→CVE-2022-32929 - Bypass iOS backup's TCC protection · theevilbit blog
• From a disinterested 3rd party→iOS Backup Passcode Prompt-iMazing