Also on the iPhone, depending on the privacy settings, Apple is masking your device name. So the BMW can get confused and just displays the Mac address instead of the name of the phone from time to time.
Have you had more than one device connected? We have two phones connected. I get this message when the other non-default device comes near and then gets out of range.
Then it's your iPhone as I indicated above, nothing to be worried about. My android phone will occasionally randomly disconnect but you can immediately re-pair it.
I think it's likely a rollover of the MAC address. I turn off Private Wi-Fi for my car's wifi, but I'm not sure it's needed. I think this message is innocuous.
There is a default Wi-Fi setting in Android that randomises the MAC (hardware) address to increase privacy - MAC address type. I have to turn this off for work networks to make sure I always get the same IP address.
Not sure how that impacts connections to the car or whether there is a similar thing in Apple phones but I have never had that message in my car.