@JeremyCherfas Sounds like maybe the photos are being saved encrypted. There's a "unlock while phone unlocked" option and a "unlock once the phone has ever been unlocked after boot" option at the system file API level. See the fileProtection… options here [developer.apple.com].
The iOS security guide PDF [apple.com] clarifies the default:
Protected Until First User Authentication (NSFileProtectionCompleteUntilFirstUserAuthentication):
>
> This class behaves in the same way as Complete Protection, except that the decrypted class key is not removed from memory when the device is locked. The protection in this class has similar properties to desktop full-volume encryption, and protects data from attacks that involve a reboot. This is the default class for all third-party app data not otherwise assigned to a Data Protection class.