No-one has said it is a systemic issue other than you. It's clearly a problem that only impacts some users, and there is no pattern yet for even the type of phone as some iPhone 8's for example, work, and others do not, even on the same iOS level.
If you're going to get pedantic, then there are more bluetooth profiles involved than two. There's HFP, MAP, AVRCP, A2DP, PBAP and maybe some others, however, it's unlikely these are the main issue as these are implemented by iOS and if they were the problem then it would be systemic. Also iOS 12 is officially NOT compatible with ICT. JLR have only certified up to iOS 11.3. However, if we wanted to just believe that then we could simply ask CRC and take their word as gospel as they are always right, and no-one ever upgrades their phone beyond what JLR certify or buys new phones not on the list. (Same for Android with Galaxy S10 etc. and there are plenty of Android phone issues too where different manufacturers can use completely different hardware...)
The problem is far more subtle than 'it's not connected to bluetooth' and the AirPlay thing comes in not because I'm saying the phone or ICT use it to talk to each other (they don't), but purely because Apple keep tweaking the rules around how apps that handle audio have to behave in order to improve their AirPlay 2 experience for end users. Apple are big on user experience rules, and constantly change them. And it's apps that are most likely to be in use on a Bluetooth audio connection, as without some kind of audio app, you get no music by default. The beauty of a bluetooth audio connection is that you can stream from say Spotify, even if the in car system doesn't support it, and at a basic level (thanks to AVRCP) you can use the steering wheel controls to skip tracks etc. even if there is no convenient display on ICT for Spotify...
To be fair, this is the kind of thing it's impossible for any dealer to diagnose. This needs a phone that is displaying the issue to go to Gaydon where the ICT team can hook it up to a test rig, and determine which profiles (if any) are unable to correctly connect between the phone and ICT as they will be able to run the ICT software in a test/debug harness. That would help enormously in narrowing down the problem. I am more than happy to take my iPhone to Gaydon for a day if anyone from their ICT team reads these forums.
On my phone (XS using iOS 12) I can clearly demonstrate that Sonos (which does not use Bluetooth) can prevent my phone from being able to connect audio via Bluetooth to ICT if it was running and the last thing I used on my phone (letting it just timeout into lock mode) before I get in the car and try and use Bluetooth for playing music. How can that happen if its as simple as only Bluetooth is in use? Hence my suspicion, and I've only called it a suspicion, that Apple have made some rule changes around audio stream usage that maybe some apps are not correctly adhering to which then causes problems. The big thing Apple have been changing recently and tweaking (with more coming in iOS 13) is Airplay2 for their Homepod ecosystem and AirPlay 2 TV control (new in a recent iOS 12 update) which is why I'm guessing this is where the problem may be. Not with Airplay as a protocol, but with rules around audio channel control and audio hand-off.
To complicate further, ICT does allow two phones to be connected at the same time, one for phone use, and another for audio... Which implies that the straight audio connection (A2DP) is not required for the handsfree phone function... which also implies the handsfree function may not use A2DP to sound the ringer... or does it? These are things only the ICT software people can confirm, but to do that successfully they need a phone that has the issue.