Hello HansCastrop,
To estimate the flight altitude value for the report Metashape performs the following operation: at first it estimates the average distance from the camera center to the sparse cloud points that have valid 2D projections on the given photo and then takes the median value for all the aligned cameras in the project.
The ground resolution value for the report is also estimated individually for each camera first as the flying altitude (like previously described) multiplied by the angular resolution of the pixel. Then the value is averaged for all cameras.
So the value can be properly considered for aerial survey projects, whereas for close range projects (like scanning monuments) it may be incorrect.