I've just come across this thread. I just got a brand new KIA EV6 and I'm having a similar issue. Passive unlock just doesn't work. I do think it's the phone and not the car as the digital key works when I open my Wallet app and tap the unlock button. Interestingly, the phone isn't always connected to the car when I'm nearby but closing the Wallet and opening it again wakes it up most of the time (though sometimes I have to repeat this procedure a couple of times). I'm using an iPhone 17 Pro Max with IOS 26.1.
I've seen a few threads describing similar issues and it does seem to work for some, on a variety of vehicles.