I assume your phone is current enough (2020+) to use FBE - file based encryption. It’s split into to realms, credential encrypted (CE) and device encrypted (DE). CE is what’s behind your PIN or gesture, DE is device derived.
Settings like the lockscreen background are stored in the DE parts, and thus available to the system without user credentials.
If you follow aibd link to Androids encryption docs, at the bottom of the FBE entry at “Storage Classes” you’ll see the paths how CE is split in the filesystem hierachy vs DE: https://source.android.com/docs/security/features/encryption/file-based