Hi Soma,
It is possible to sort using Earliest start date. You need to define and apply in sorting.
Expand the first part of screen, to view the dates of your WBS elements. Since i dont see any thing wrong with the screenshot you have mentioned.
Screen shot : 1
Activity 50 starts 1st - 10 - 30 - 20 - 40
If you use earliest start date, then this should how it displayed, so system is correct
Regards
Terence