It works but now I have another issue : if I click on the group header or expand / collapse the group, it displays the original value and not the label I put.
The most bizarre thing is that the label comes back when I click on another cell.
Screenshot 43 : initial state
Screenshot 44 : group header is selected, the label is replaced by the original value
Screenshot 45 : re-selecting an element gives me back my label
My code is located in the ViewCellFormatting event handler and is this: