After experimenting further, I found out that the proper statement is "top to bottom direction" and it works correctly.
Tested on this example:
@startmindmap
top to bottom direction
* Debian
** Ubuntu
*** Linux Mint
*** Kubuntu
*** Lubuntu
*** KDE Neon
** LMDE
** SolydXK
** SteamOS
** Raspbian with a very long name
*** <s>Raspmbc</s> => OSMC
*** <s>Raspyfi</s> => Volumio
@endmindmap
And Mindmap is shown as expected.
Can be closed.