I have created drop-in example code to demonstrate.
Add the contents of the attachment to the examples folder and navigate to .../ext/examples/custom-renderer-issue/hello.html
Click the button to launch the window, and then the button to bill all the items (copy from the bottom grid to the top editor grid).
Edit the value in the last column, leave that cell and note that the new value does not appear.
The value in the store is changed but the renderer has not fired.
Click the button to remove all the items, then re-add them, to verify that the value in the store has changed. (or simply double click to edit the value again)
EDIT: I have seen that this is similar (maybe a dupe of) http://extjs.com/forum/showthread.php?t=33791, but that has no replies yet.