Elevation Corrections are calculated with data from professional surveys instead of the data from your device. If your device has a barometric altimeter, Elevation Corrections are disabled by default.

When corrections to elevation data are made, each trackpoint of your activity contains the elevation from the web service, not the elevation provided by your GPS device.

When an activity is recorded with a non-barometric altimeter enabled device, Elevation Corrections are enabled by default when uploaded to Garmin Connect. If your device has a barometric altimeter, Elevation Corrections are disabled by default. You can enable or disable this feature for each individual activity that is uploaded.

When Elevation Corrections are enabled, each trackpoint of your activity is updated to the elevation data from professional surveys instead of the elevation provided by your GPS device."

Then why do you run elevation corrections to when creating course from device that has barometric altimeter and corrections disabled?

To fix your elevation corrections I would suggest the following steps:

1. Admit that your elevation corrections data is bad and give as global options to disable elevation corrections on activities and on courses.

2. Get better surveys data! You can example use this as your starting point: https://www.gpsvisualizer.com/DEM_coverage.php 


Ps. Has anyone ever seen Garmin's rep answer anything in any of these Garmin Connect Web forum posts? Is it really pointless to talk anything to bugs/improvements here?