Artwork question

Hello, i have replaced several artworks yesterday on my NAS, everything was fine on Mac, iOS and Apple TV.
Today i opend up the iOS App, outside from home. So no connection to NAS or my lokal NAS. Then the pictures started to fall back to the original ones??
Is that normal? I thought they are stored in my cloud storage.
The all started to change, then i closed the App instantly.

Still in the “planned” category, here’s the suggestion thread.