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Probably no way for users to disprove this without access to iCloud servers, but users could prove it if they are getting photos reappearing from a previous phone, since obviously those could not be from local storage.

I think a reasonable counterargument to this is a theoretical internal outage at iCloud and one or more machines storing data needed to be restored from backups. The backups contained "deleted" photos from before users deleted them.

Could be some kind of syncing consistency bug between servers too.

It's possible, but there is a lot of surface area for sync bugs before you get to "cloud provider must be harvesting our nudes" territory.