I have the same issue indeed with Strava, hopefully HMD is fixing this soon.
I have the same issue with Nokia 8S / Oreo and Sportractive, Strava or Runkeeper. Having Amazon Music Player running at the same time fixes the issue. But without a second application running, it does not work. All OS adjustments that are reasonable do not fix the issue either.
Does Nokia / HMD already look into this issue? It is clear that a long battery time is a key interest of device makers, and GPS does consume a lot energy. BUT it is my decision to use it for my activity purposes and hence I know that the battery will be strained, but this is ok in this time. My decision, isn't it?
For me the May update (2018.05) fixed the issue with Strava.
Same with my Nokia 8 with Oreo 8.1.0 and all the latest updates
Tried with deactivated com.evenwell.powersaving.g3.overlay.base.s600ww and com.evenwell.PowerMonitor.overlay.base.s600ww but nothing helps
An update on GPS problem with Nokia 6.1. Normally Sportractive does not work. However, when I use Wake Lock to get a partial lock I could successfully track my runs from short 45 minutes to medium about 3 hours successfully.
One curious thing I noticed is that I need to get the partial wake lock before I get the GPS lock for the first time. Need confirmation from other users.