I my be wrong but I think myChevrolet and Car Play are unrelated. I haven’t installed 13beta but expect the problem with this app has more to do with the app itself than with CP in particular. I’m not defending GM’s CP implementation in the car, I just don’t think you can blame the mychevy not working on CP bits in the car.
If CP will just connect reliably and stay connected with iOS13 it will be a win in my book. Hoping GM will update the software in the Bolt is probably a vain hope. As I said in a previous post, the only good thing about CP in the Bolt is that it’s better than nothing - usually.