Arthur, has your unit gone offline recently? If this happened to you you might see that when you tap on the Song, Album, Album Artist icons are ghosted out.
If nothing else works you could try deleting the album or song in your playlist that doesn't display art and then putting it back in. This can be a pain but it has worked for me.
I bring the U1 back and forth between Austin and Chicago. Despite the fact that the two NAS in each location are identical it gives the Lumin fits and most if not all of my artwork in my playlists disappear when I move it for one place to the other.
I have found that deleting the song or album with the lost artwork and then re-installing it has mostly worked for me.
As a curiosity when my artwork has gone poof, it doesn't happen to any of the songs that are from Tidal, only the ones from my NAS.
One nice feature that Aurender has is that it will identify which songs or albums are from Tidal with a tiny icon in the corner. You can't really tell in Lumin which is from where until your artwork goes away.
Before you try my suggestion make sure that your Lumin is online.
As a housekeeping measure I will do a periodic minimServer rescan or update music library under MinimServer options.
I'm no computer audio expert. But I have found that this has worked for me when the artwork disappears and the suggestion that Peter has made to me have not retrieved missing artwork in my particular case.
Also check and see if your NAS is asking for any updates under its control panel.
You might a check in with the MinimServer community board. It is a wealth of information.
Please let us know how it turns out.