Good catch.
The attached patch fixes this. The solution is just adding the wxRB_GROUP flag to one of the radio buttons, but the patch has grown a lot due to wxSmith sorting things. It also changed automatically FlexGridSizer1->Add(0,0,1, wxEXPAND, 5) to FlexGridSizer1->Add(-1,-1,1, wxEXPAND, 5), and this moved the radio buttons to the left.
EDIT: the moving of radio buttons is fixed in the next message's patch