I have the same problem. Using iOS beta 14.5 and WatchOS beta 7.4 Hope for a fix soon :)

After updating to public beta 14.5 beta 2 and WatchOs 7.4 beta 2 it seems like it has finally been solved. I just installed both, called my phone, all was fine. Went out to my car and connected CarPlay, then back inside and called my phone again - and it still works!! So I hope it wasn't just a lucky moment, but keeps working :)