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Interesting one. It's unlikely to be a GPS issue as a GPS receiving device has a lock on three satellites but has no way of knowing if other devices in the vicinity are also locked on the same three satellites. I've definitely recorded rides with two separate devices before, occasionally more (used to work for a sports device vendor so did a lot of pre-release testing).
It could possibly be the Strava or Garmin apps, but seems unlikely.
I'd reboot the Garmin and see if I can get it to record distance first.
Random question - is the fact that my garmin isn't recording distance in any way connected to the fact that I was also recording on strava on my phone? Happened both ways of my journey yesterday.