The bug can be reproducible with Garmin Connect for Android 3.12.0.1. Not with iOS version.
When I share an activity from Garmin Connect from 3-dot menu -> Share, it appends unnecessary querystring "share_unique_id" into sharing URL. For example, connect.garmin.com/.../<activity_id>.
This querystring brings us to an incomplete activity page as the screenshot below.
While in iOS version, the URL isn't appended with share_unique_id and it brings to complete activity page.