Not sure if it's intended, but it seems that the Grid's RowWidget plugin is mapped to the row index, instead of the record.

Scenario:

https://fiddle.sencha.com/#view/editor&fiddle/2vj1

Expand the first row. You'll see the first inner panel contains the same title as the column "name" from the main record. Now if you sort by "Name" and open the new first row, a panel with the same name will be created. If you play around with the column filters you'll arrive at the same bug.