Any Strava user here? Start getting mismatch numbers from Garmin Fenix 5X and Strava

Yup, that's a big difference. Timewise, is it perhaps due to the difference between elapsed time and moving time? (Not that that helps explain the distance discrepancy) Were you using normal GPS or perhaps Ultratrack? (Ultratrack is terrible for accuracy). I know that when I first tried out mine there were some very large issues with distance, but they seem to go away. Perhaps look at how Strava calculates distance. It clearly doesn't just accept what the watch sends to it. Also see if you can work out what the distance actually was (using a map independent of Garmin and Strava).

Another thing is that I sometimes am suspicious of how the watch calculates the distance because I do remember also once uploading a track from the watch to Basecamp and having it work out the distance. It got a total quite different from what the watch was reporting. Again, I haven't noticed such problems lately.

Also, check your GPS firmware version in settings->system->about->page down a couple. Has it properly updated to 2.30 or is it stuck in 4.40 (!) ?

Sorry, not much help here, other than to say, yes, there is something odd going on. I guess you've looked at other reports on the forum about the lack of accuracy from the GPS.