I would have to agree that the problem is somewhere within iTunes, which then manifests itself differently on different versions of the remote app, depending on what iOS device you run remote on, and what version of iOS you are using. For example, I am running an old version of the remote app on iOS 5 on an iPod Touch 3rd gen. It does not have the exact same problem as my iPad 2 with latest iOS and latest Remote app, but it still does not display everything that it used to. It has that "getting stuck" feeling to it, similar to the iPad, but not exactly the same. The iPad actually works a little better. And yes - my "iTunes fix" (keep Preferences dialog open in iTunes) does work for the old remote app on the iPod touch as well.
Another thing I discovered is that this issue may not be limited to the way that iTunes communicates with the remote app. For example, the other day, I was streaming my iTunes music to my Apple TV. The artwork would intermittently not display on the Apple TV! Sometimes it would, sometimes it would not. (And yes - I double checked that the track had artwork on it by looking at iTunes on the PC while that track was playing.)
Then I tried my "trick" of keeping the preferences dialog open in iTunes. (As stated previously and confirmed by Nightwolf67, this has the effect of keeping the communications to/from iTunes flowing when using the remote app,) i.e. remote app does not get "stuck" when displaying artwork. In this case, the trick also "fixed" the problem of the missing artwork on the Apple TV!
Note that I was not playing the music FROM the Apple TV by accessing Computers and then my iTunes library, but rather playing the music in iTunes itself, and then selecting the Apple TV as remote speakers.
I wish Apple would take a closer look at the iTunes code and see if they can recreate the problem (should not be hard), and then fix it. I guess there must be so few people out there using the remote app to control iTunes that this issue has never really gotten that much attention from Apple.