A significant amount of album covers is missing after the most recent update. This is happening on the PC version but is OK on the iPad/iPhone versions. The artwork shows up when you select the album itself. this affects both my local library and Qobuz.
You could try clearing the local image cache via Settings/Setup. Of course that might mean that you lose all your artwork, but it’s perhaps worth a go.
Have you rebooted the Roon server?
Clearing the Local Cashe worked. I think I accidently interrupted this previously which caused the problem. Thanks to Giles_W for triggering my aging memory! 