ASL Altitude inaccurate in flight plan

When I check the altitude of a few mountain peaks in my sorroundings the number is about 300m too low compared to the Actual Altitude above sea level.

e.g. Hover over the peak of the Mount Everest with your curser: It shows about 8500 Meters. It's actually 8848m ASL.

In google earth Pro it seems to be right with an Accuracy of 1m.

Am I missing something?

Start by reproducing the altitude discrepancy in the flight plan by hovering over Mount Everest or another mountain peak, then compare Mission Planner's ASL value with the actual elevation reported in Google Earth Pro. The issue is complete when the source of the roughly 300 m difference is identified and the displayed altitude is corrected or the limitation is documented.

Written by the indexing model from the issue text.