Yes, at first glance it's the same for me, but if you enlarge the dialog (make it higher) to a certain level, the buttons appear below the abbreviation list.
I had a similar problem in an application with a wxListBox in a wxFlexGridSizer. I thought, due to the listbox' wxEXPAND flag, it wasn't nessecary to set a size, but after specifying a size for the wxListBox object, it didn't enlarge the sizer anymore.