I too have the same problem with the iPhone X in my 2012 Mazda 3 MPS since December, however mine will connect, allowing me to play music and take calls, but the connection will sporadically drop out and reconnect every few minutes (or seconds). No iOS updates have resolved it, and my $1500 phone needs to use an AUX cable.
I've taken it to an official Mazda service shop, and they had no problems connecting it to an iPhone 7 (iOS11). I also have no problem connecting it to my iPhone 6.
2012 Mazda's use Bluetooth v1.0, whereas the new iPhone have (for the first time) Bluetooth v5.0. Apple does not communicate this incompatibility issue to their customers before purchase, which is disgusting and a big problem.
The fact that our devices CAN connect and work, means it is software related, which Apple should be able to fix. There are no Mazda firmware updates available – which clearly aren't the problem anyway since they still work will all other Apple devices. iPhone X's should be able to communicate to ALL versions of Bluetooth, without their customers having to spend another $1000 for an aftermarket Apple Carplay system for their cars.
Fix your software Apple.