This might not even be a Garmin issue. I’m using a Fenix 7x Solar. The last several runs I’ve noticed a huge difference on elevation gain between Garmin Connect and Strava. It’s usually a big difference. Garmin Connect is showing the correct elevation cause I’ve done these runs dozens of times using a Fenix 6x and Forerunner 945. Strava doesn’t even give me the option to try and elevation correction.