My workaround was successful… Exporting the GPX straight from Strava into GARMIN\NewFiles on a route was fine. Exporting Strava->Connect->Edge resulted in ClimbPro being out by ~300m after 10km. The turn-by-turn navigation was still spot-on.
For whatever reason it takes me 23 mins to explain and demonstrate this in a video. 
Hopefully it’ll prompt Garmin to look into the issue. Even if the source of the problem is Strava, they should be able to handle it and not have their users have an extremely shitty time with their products. (and that’s putting it lightly… climb #2 in my video completes on one Edge before it really begins on another!)