Problem in Sorting Activities

Hi

working with PS,  have a problem in sorting activities.

I have a project with this structure:

As it's showing in the picture, the activity "20" is the successor of activity "30".

I need to sort the activities based on Earliest start date. when I do this, the activities are sorting as below:

As it's clear. in graphic view, the activity "20" is now under WBS1-2 that it's wrong.

of course that is just in view, in activity detail screen nothing changed and the activity belongs to WBS1-1.

I need a way to sort this activities without change in WBS. (I need a sorting based on WBS element and earliest start date.)

Thanks

Best Regards