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.

So glad I don’t work for Apple Support anymore. Got a lot of these types of calls. It was easier to investigate on a Mac, but iOS could be stubborn.

Large “system” storage can be anything, and it always changes from major update to major update. It could be important data that isn’t being categorized properly, or garbage cache that isn’t clearing like it should.

Typically, Apple keeps an eye on this sort of thing and provides guidance on what it most likely could be, and which steps to start with when troubleshooting this symptom.

Call Apple support if you want to try to “fix” this, but be ready for lengthy and potentially invasive steps. Or just wait until it sorts itself out after an update.