As I found out later, with a small test, when I read in the properties
dialog that base level is basement, top level is groundfloor, and have the
"top is attached" checked,
it is possible that the top is attached to another level and not the top
level. Still, you only see the level you used to create the wall, and not
the new one when attached. What is more weird is that the height of the wall
is also the initial. For example it says 3m instead of 6m it should , since
the wall now goes to 2nd floor