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Same happened to me w my iPhone 6S but it stopped doing that after a week or two.I have a Samsung Galaxy S8 and have only had my M3 for a week and a half and have had 2 issues. One time it would not let me open the door and this morning the doors unlocked but it asked me to put my key card on the console to start. I had to use the airplane mode on/off trick and it came right back.
I will add one follow up. My Pixel 2XL has never failed to open the car, but I did notice something recently. I generally have the phone in my hand or in a jacket pocket and it always works fine. The other day I was carrying a Microsoft Surface and the phone together in one hand. When I got to the car, it wasn't unlocked which surprised me. The way I was carrying the phone and Surface placed the Surface between the car and the phone. As soon as I rotated my hand so that the phone had line of sight to the car, it was unlocked and ready to go. So, obstructions between the car and your phone, including your body most likely, could prevent the car from opening even if they are very close.
My wife and I both have pixel 2 and it's annoying. Works in my pocket or her purse about 10% of the time. Works 80% if I take phone out of pocket and the screen turns on (phone still locked) 10% it just doesn't care at all and I have to use the notification to do an unlock.
Update to my own post, finally just gave in and got rid of the Pixel 2 (non-xl). I'm afraid to jinx it, but I got a Pixel 3 (non-xl) and it's worked every time in the last 2 days. In my back pocket, doesn't matter which door, it's been perfect. It's like magic...