I've rented beamers with Sixt at LAX and O'Hare, the latter just a couple of weeks ago. I didn't have any issues connecting to bluetooth so that I could use CarPlay. Could it be inconsistencies in how their BMW fleet is set up?

I ususally set up the bluetooth connection before driving off the lot, just so I can get staff assistance if it doesn't work - not being able to connect to the car is enough for me to insist on a new vehicle if they're unable to fix it.