The Station or track artwork should be appearing almost instantly.

I suspect there is some latency with your controller updating.

If you force close the app and reload, does the applicable artwork appear when the controller reconnects?

Try a reboot of your controller to see if that helps.

Does it work fine on another controller?

No, that has nothing to do with it. I has to do with using SVC to do any call outs (Apple Music or Sonos Radio HD) relating to stations. All controllers look exactly the same. I did the reboot as well. 

I no longer subscribe to Sonos Radio HD, but with Sonos Radio, started via SVC, the applicable image appears instantly. Same with Apple Music Stations.

This is what I get in My Sonos. The one on left and right are SVC callouts, the one in the middle was chosen using the app. All are Sonos Radio just captured a little but the same holds true with Apple when I asked for “play alternative.”  

Ah, okay got you now.

Yes, My Sonos also shows the same grey boxes for Stations started by SVC

Dont know if that’s intended behaviour or not…