Missing iCloud Data
Why are some pictures and photos are not backed up in iCloud?
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What does iCloud back up? - Apple Support
iCloud backup doesn’t include information that is synced and stored in iCloud, like Contacts, Calendars, Bookmarks, Mail*, Notes, Voice Memos, Messages*, shared photos, iCloud Photos*, Health data*, call history*, and files* you store in iCloud Drive.
*-If you had turned on Messages in iCloud, iCloud Photos, Mail, Health data, call history under iCloud, and files you store in iCloud Drive your content is automatically stored in iCloud and is available on all devices that use the same Apple ID