Well that tells me it’s likely syncing data from iCloud then. Or from some other cloud storage/sync/backup service.

Have you checked your recently deleted photos album? I vaguely remember that causing uncatagorized storage issues in the past.

If you still want further help, you’ll need to call them back and speak with a senior advisor. But again, you’ll be trying different steps to see what eventually resolves it. There’s no way of knowing for sure without an engineering escalation, which requires more testing and data collecting, and waiting for a response. Holidays are slow, so good luck.