Hi Bernd @bguenthe ,
No, it’s not a bug!
It’s a classic beginner mistake; your model, most likely has so-called ‘double Vertices’. You probably Extruded or Inset the top Face and then Cancelled the Move part.
The solution is to Select everything and the Merge the Vertices: press A and then M > Merge > By Distance.
If that doesn’t help, with everything still Selected, press SHIFT+N, to re-calculate the Normals.
But my money is on the double Verts.
