Yeah the Download Album Art Manual option for a single song part resolves zero artwork for any of my 1,000's of songs now too. It's the only way i have downloaded album art prior to a week or 2 ago when this started happening. Havent found a fix yet, but found myself having the same issue now sadly. No changes to songs, or anything and first time experiencing this ever. No FiiO device.
Will post when i find a fix