well so far since i got the 28.1 update earlier i can say its been more consistent because i had the same issue with the 26 update. I was about 50% success. So far on 28.1 the 5 times i have went to my car today it unlocked every time, but you know mileage and depending on phone type this may very. Im running iPhone X on iOS 12 beta 6.
That's really interesting. I'm also on an iPhone X w/ iOS 12 and I've only ever had one phone key failure in 2.5 months of ownership.