Prism targeted network communication to my knowledge, hence the data wouldn't be siphoned from at rest encrypted devices. Instead it would've been leaked before it was copied to that local encrypted device, whenever it was transmitted over the wire. Eg when your background task uploaded it to iCloud or similar.

It’s worth remembering that since Snowden, much of iCloud is now end-to-end encrypted using keys that Apple cannot unwrap: https://support.apple.com/guide/security/secure-icloud-keych...