I use MapMyWalk on my iPhone X to keep track of my walks. The routes I take are along local streets. There aren't any major overhead obstructions. Everything was fine on 11.2.6. On the same well known routes my course is wildly inaccurate on 11.3. The course will jump several blocks from the real course.
Since I seemed to be the only one with the problem, I decided it probably wasn't iOS causing it. And because it started with the update a restore might fix it so I restored 11.3 and everything seems back to normal.
I noticed this too the other day and was wondering why. Before the update I never experienced that even on the rainy or cloudy day. --- Post Merged, Apr 14, 2018 --- It was pretty dangerous driving on unfamiliar road.
GPS issues can be hardware - I had an issue with my iPhone 6 which was intermittent and fixed by replacing an internal antenna. Just bear that in mind if you see it again - software update/restore having an effect might be coincidental.
Every time I hear about these issues I wonder if the GPS receiver hardware on the phone is having problems. When GPS cannot be used, the phone falls back to the WiFi location database and cellular triangulation, which are far less accurate and will cause that "jumping around" effect you are experiencing.