How is this setting configured?
What is the result in the Total Ascent field on Garmin Connect?
There are 101 methods to calculate elevation depending on the settings and the accuracy of barometric sensors. If baro-pressure is not used (because the device doesn’t have a pressure sensor or because it isn’t working correctly anymore), elevation will be calculated by data from online available maps. If GPS reception was mediocre during the activity, you get elevation data from points next to the road iso points on the road.
The GAP algorithm in Intervals is the same as the one from Strava, but almost surely, the platforms don’t use the same online maps.
If your device has a correctly working barometric sensor, the above setting should be set to Accurate Elevation = Yes and ‘Use Elevation Correction’ unchecked.