I am also experiencing this problem, but can add some specificics to it from my experiences that may help (or annoy others). First, I have two cars with CarPlay -- the first is a 2018 Ford F150 Raptor - this headunit is the factory unit with Ford Sync 3 and uses a USB wire to connect to the system, the 2nd car is a 2019 Mini Countryman SE Hybrid -- carplay in this car works with a wireless adhoc wifi setup -- similar to that of BWM and other manufacturers employing the new wireless carplay tech. One thing to note here is that there is no option to use a USB wire in the MINI -- only wireless. In all the things that I describe below, I can replicate them on both systems which I feel is a pretty good control for the USB cable not being the culprit. I also have an iphone X, an iphone 6s plus, and an iphone 6s that I confirm findings with.
Anyway, as I said above, I also get this problem with Pandora, but I can also replicate it with just about any audio streaming app except for Apple Music. Apple Music seems to always work perfectly and the graphics always load regardless if I had it open or not previously on all three phones. This is annoying as I am already paying for Google Play Music and Amazon Unlimited Music -- I do not want to have a 3rd ongoing bill to listen to the same music...
In addition to Pandora, I can also replicate this lack of loading graphics in almost the same way with Amazon Music, Spotify, Google Play Music, Sirius XM, and an app called CloudBeats. Because these apps all behave the same way, I am pretty convinced this problem is specific to Carplay and not the apps (although it could be related to the API that the developers use compared to the one that Apple uses for Apple Music).
Some other apps I use behave differently. One in particular is Overcast. This app almost always loads the graphics to load media on the car screen. I am not sure what is difference between Overcast and the above apps. The media selection screen appears very similar. Another thing that is VERY weird is that Amazon Music often times freaks out and streams music via the speaker phone function (rather than A2DP). Usually, a solution to fix this is to open another streaming audio app, then re-open Amazon music and the music is streamed correctly.
I have been able to replicate all of these issues with both cars (wired and wireless) and on all three phones. Rebooting fixes the issue 99.9% of the time and force closing the apps also seems to work consistently to fix the issue (we shouldn't need to pick up our phones while driving as a matter of safety). The thing that I am most curious about and wonder if other users see the same is with respect to Apple Music almost always working correctly. It seems that Apple Music is using a slightly different version of the API/dev tools as the interface, including the menus and positioning of some buttons is slightly different from other apps. Could this be something that fixes this issue? I don't know, but I am curious what others think. Not having to pick up my phone and interact with it while I am driving is THE reason I paid the extra money for CarPlay in these cars. Basically at this point, unless I use Apple Music, I am back to interacting with the phone while I drive.