iPhone X here...
GPS is definitively having some issues.
Usually indoors (in my house), I'm getting at least 16 meters accuracy after 20 secs max... now maximum 33m after 3 minutes.
on the highway, I'm usually at 8 meters... the best today was 17m... and sometimes it wasn't even locking to GPS.
when not moving, I usually get 3-4 meters accuracy on a perfect clear sky... now max is 17 meters.
I did a reset network settings... which sometimes fixed the problem on previous version... but it does absolutely nothing on the iOS 12 beta 1.
went back to 11.4.1 beta 1... and all is fine.
I did not even reset anything... just "upgraded" to 11.4.1 through iTunes... and pretty much everything is there.