I did an off road ride of about 50k and recorded it on both Viewranger and Strava. Both recorded the distance almost exactly the same but there was a massive difference in the height gain.
Strava said 800m and Viewranger said 1200m.
Does Strava under-state elevation or does Viewranger over-state it? Or both?

Was the height recorded by your device or calculated by the app afterwards, and do both apps do it the same way? I’ve seen the elevation profile of the same lap of a 24 race change considerably overnight due to pressure changes. I know Strava can correct elevations if it knows the contours of where you’re riding, no idea bout ViewRanger though.