When a program calls a file picker with the ShowDirsOnly option, if the startdir is at least the third starting from root (for example: /home/user/xxx), this startdir is well selected, but user can't see it directly in dialog, he has to drop down to this folder.

An easy way to see this is to use kdialog (>=5).
just type:
kdialog --getexistingdirectory /home/$USER/Documents

Note that it's not a bug from kdialog because this affects all program calling kde file pickers, I tried it with python3-qt5 and didn't find any workaround.

but if startdir is /home/user or usr/share or anything else far from 2 folders from  the root, it's well.

I show that on neon, and on kubuntu 16.04 with kubuntu backports (not with kdialog on kubuntu because kdialog here uses Qt4).