Very, very similar issues here. I bought the iPhone 8 and have had a heck of a time getting an accurate GPS signal. Waze/Gmaps would start out fine, and then quickly lose me about two minutes into the drive. The signal would periodically get picked back up, only to promptly be lost again. Closing the apps and restarting periodically got the GPS going again, but not before quickly crapping out. Needless to say, this is no way to navigate to a place you haven't been to before, and completely unacceptable for a brand-new phone. I spent plenty of time on the phone with Apple tech support -- who were helpful and sympathetic -- but we exhausted every option (reset settings, location, LTE, data and talk, bluetooth, airplane mode, wipe the phone and stat anew) and absolutely nothing worked. They sent me to the Apple store for a replacement device.
Lo and behold, the even newer iPhone 8 still seemed to be giving me problems with accurately finding my location while in my car on GPS. I couldn't believe my bad luck, and started wondering if somehow it had something to do with my car, or where my phone was positioned in my car, because I'd also used the GPS with my running apps and hadn't had any issues.
So this might be the strangest part: I've been using an Alisky car phone mount, and I finally took the phone out of the mount during my drive yesterday, and suddenly GPS appeared to be working again just fine. It navigated me all the way home, and so then I tried it again -- outside the phone cradle -- and while the GPS seemed to lose me once or twice, it also picked me back up within about 30 seconds instead of 5 minutes, and fully navigated my entire route.
I at a bit of a loss to explain this -- the rational person in me can't fathom how a piece of plastic could have any effect on a GPS signal. Is it possible that the GPS chip in the iPhone 8 was moved in such a way that depending on the position of my iPhone in the Alisky car phone mount it could actually block the GPS signal? Seems preposterous, I know, but I am really coming up short for an explanation as to why the GPS has mostly worked since I removed my phone from the car mount. Could it be the combination of the silicon Apple-brand case I use plus the plastic Alisky holder is somehow interfering with the GPS triangulation?
I've got some more experimenting to do before declaring this now 2nd iPhone 8's GPS to be busted, but I'd be curious as to whether anyone else has had a similar experience.
Or do we all think this is an iOS issue? If so, I don't know how Apple hasn't addressed this yet as it seems like a fair amount of users are having this same GPS problem.