I can provide code if necessary but it happens just creating a new panel with a tab and the grid inside, using store as MemoryStorage, with data defined in the store (inline).
The problem should be fixed in the same environment, so even if I try to make it work in your example it is not the way to test because I use Sencha Architect in a MVC application, and the layout is not the same.
But it works in a non-mvc example without Sencha Architect, probably the problem happens only when using Sencha Architect.