You are re-open the edit form based on index so it is opening the wrong item/row in edit mode.
What I would suggest is that when you click on edit button or after opening the edit form please store current row datakeyvalue in variable and after refreshing the grid based on variable's value find the particuler row and open it in editable mode.
Please check below links for to get understanding of clientdatakeyvalues.
Extracting Key Values Client-side
Access RadGrid Row and Column on Client Side