I had this issue after upgrading to an iPhone 14 Pro from an iPhone 12 Pro using iOS 16.0.2.
I tried several restores from various backups.
Always ended up wtith system data of over 100GB within 24 hours of a restore.
Powering the phone off and on would wipe the system data but it would then slowly grow back to +100GB within 24 hours.
FIX: As mentioned on here by someone else: Logout of icloud on your phone. Power the phone off and on again. This has fixed the issue for me.
Moose290XX
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