I've logged this with Garmin support, many many months ago but seems to get no traction.
The Course planner I use heavily for Trail runs, is crucial that the total Ascent is accurate to understand how much water and how hard the run will be.
Also, other metrics like Pace Pro will be off because it thinks a climb is a lot less than it is in reality.
Some examples I have come across.:
Planned this run, Garmin Course planner says total Ascent is 565 m
After the run:
911m Elevation Correction. 1,007 m watch recorded (Strava also matches what the watch and corrections come up with)
As you can see, almost 400M more in real ascent, and did I feel it, really struggled and not prepared for this (coming back from injury)
Another example:
117M total Ascent in the Garmin Course Planner
Actuality it was 406m Elevation Correction. 368m watch recorded.
Every Course I've panned is always under, and by a significant amount.
Longer runs 20km + just get worse...
What baffles me, is the course planner is under measuring the ascent.
But running the course and uploading to Garmin Connect, and using their own Elevation correction, (usually very close to what the watch records) is so different than the course planner.
I just hope Garmin if you are listening can fix this, it becomes a safety issue when planning longer runs and you think the elevation according to you Course Planner is far less than what it actually is. Makes a massive difference....