Upgrade your vehicle dashboard firmware, and try again. How that happens depends on the vehicle. One vehicle I just upgraded uses a specially-formatted USB flash drive, and the manufacturer provides directions for downloading the firmware and building and using the USB flash drive. Others might involve the dealer.
Magnets aren’t involved with Bluetooth nor with Bluetooth interference. Not unless you’re whirling the magnets quite fast to generate electrical power, that is. And you’re not.
Bluetooth is a short-range protocol, and I would not expect any issues within a radius of 10 meters, absent interference.
As for the first question, I was asking whether you’re testing both iPhones with the same car, and that does appear to be the case here.