editable grid with GwtRpcDataSource writes old value when using grouping

version: v8.3p_2013-09-21/Pro Deployment (built 2013-09-21)
Tested on Chrome and firefox
Using the GwtRpcDataSource

I have a editable grid that works fine when no grouping is enabled.
The moment I enable grouping the fields don't get updated anymore.
I walked through the code and stripped out everything untill I just call the server and return the data unaltered.

The value get's cleared/reset to the old value after the call to:
processResponse (requestId, response);

The grid is working fine when grouping is switched off