This seems to be a bug with the api v2, which was just recently activated for the Explore page.
Specifically, searches where the radius is at least 1 work as expected, but searches where the radius is less than 1 don’t work correctly.
Compare
https://api.inaturalist.org/v2/observations/species_counts?lat=51.30376895574157&lng=-0.18203753804374667&radius=1&user_id=jamie-aa (0 results)
to
https://api.inaturalist.org/v2/observations/species_counts?lat=51.30376895574157&lng=-0.18203753804374667&radius=0.9&user_id=jamie-aa (1594 results)
api v1 didn’t have this problem, so when Explore used v1, it wasn’t an issue
https://api.inaturalist.org/v1/observations/species_counts?lat=51.30376895574157&lng=-0.18203753804374667&radius=0.9&user_id=jamie-aa (0 results, as expected)