Same issue on the iPhone 16 Pro Max. I've been dealing with it since updating to iOS 26, and it's only gotten worse with 26.1. The Genius Bar insists it's 3rd-party software related (with no supporting evidence) and tells me that my issue connecting with my Tesla is Tesla's problem. Um...no.
I've reformatted the phone and reinstalled from older backups and I've done a clean reinstall that took me HOURS to replace every one of my apps and the related data. I've forgotten all Bluetooth devices (multiple times) and reconnected each one; I've turned off Bluetooth and turned it back on; I've restarted my iPhone (multiple times a day at this point), which sometimes gets the connections working for a short time.
Yesterday my iPhone threw up all over my Tesla connection while I was driving. Tesla gave me the "key not detected warning" meaning I wouldn't be able to drive again once I completed my current trip unless I had a key. Thank goodness I've gotten to the point where I no longer trust my iPhone and have started carrying a key card. I can't even use my Apple Watch as a backup key because its access keeps getting hosed every time I reformat my phone and reinstall the operating system or restore. I'm exhausted from trying.
The fact that Apple is ignoring this issue is completely unacceptable. I have another Genius Bar appointment tomorrow, but I'm not hoping for much in the way of resolution.