Distance Markers wrong at routeplaner straight line

During doing a rough planing of a trip with straight lines (manual) in the routeplaner, I realized that the blue distance markers show unplausible values.

At one straight line, I would expect that at half of the line, the distance marker shows about half of the value of the whole line.

If I zoom in on the line, the expected values show other unexpected/unplausible values.

See example screenshots of a line from Munich to Praha (about 300km, checked with Google Earth) where half of the line (about the border Germany/Czech) is expected to show about 150km.