Even more weirdness going on with iOS 12. I still don't have updated maps. Today I went for a drive and Maps wouldn't go fully into navigate mode. I pressed Go, and I got a scrollable list of directions at the very top. Instead of the arrow, I had a blue dot, showing where I was. The directions would not update as I drove, I had to manually swipe after each direction had passed. It was very very strange. It felt like running Maps on an iPod touch or something where it has no GPS so it was running on cached data. But the dot did update in real time. I ended up rebooting the phone, and then navigation worked correctly again (meaning I got the arrow back and it was true turn-by-turn. The swipeable stuff was gone.)
Except then the location was consistently off. Like I had a turn coming up in probably 1/4 mile, and it said "0 ft". And then when I got home, it still thought I was 250 ft away. I ended the route, and then hit the Home button to go to another app and I got the blue bar prompting me to return to navigation. This persisted for a few minutes.
Something was really really off.