Perhaps this is user error, but if not….very confusing Ux:

If I click “Run” on a different shoe, and then go back to this shoe, its unchecked. I had no idea that this checkbox was how I decide which shoe Strava chooses when I finish a run.
Each gear category should have an order of preference for that piece of gear, not a “Default Sports” checkbox that puts a piece of gear at the top of the list for a given category. Anybody who uses this app enough to pay for it almost certainly has different shoes for each of these categories.
Or am missing something?