The sort-order work as intended, the bug is not related to the sorting - only to including a walk option or not. WALKING ALL THE WAY (and other street modes) is always shown as the first option. The reason is that the rest of the itineraries is shown in a "timetable" like view. For example if
arriveBy=false then all transit itineraries are listed by the arrival time. The reason street only modes are sorted before transit is that street only modes can be time-shifted, while transit can not.
Thomas