After some debugging I was able to reproduce your problem. It lays down to the fact that RadGridView is in the second tab and the DomainDataSource is on top of the tab control. In this case the DomainDataSource has already loaded its item when the grid is displayed, which triggers a bug into the grid and it cannot correctly resolve its items source. For example if you put the DomainDataSource in the tab of the grid it will work as expected.
The good news is that we are currently working on a fix for this issue that will be available in the next latest internal build. Thank you for reporting this problem. I have updated your Telerik points.
the Telerik team