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?