Same here with X on 11.2.1.
It just started happening last night after I visited a friend who lives 37 miles away. I used Apple Maps for guidance and it was working. However after getting home, I noticed that my geofenced alerts kept thinking I was leaving and returning home while I was sitting stationary.
It's continuing today with the phone thinking I'm about 2 miles away for a split second when I open Apple Maps or Google Maps. Then zeroing in after a few seconds. When I close maps it starts to drift again. This is incredibly annoying.