I have the same problem
I have samea problem with peugeot 5008. Early mobile (huawei p20) working without any problem.
I also have this problem. S21 hyundai i30N performance. Won't connect most of the time. Isn't recognised when I plug it in. If it does connect it won't then reconnect next time it is plugged in. Apple car play works seamlessly on the same car and my kids take the **** out of me for having an android phone.
I experience the same symptoms on my S21 5G and a 2020 VW Touran with Android Auto. Samsung A71 worked perfectly, cables have been changed to rule out this option. As I've seen in the forums many S21 owners experience the same issue. All the latest updates have been installed. I'll try the phone reset and cross my fingers.