As per my understanding, you are expecting the default transition of Stateflow Chart to enter in your specified state.
Adding a new transition or new behaviour for the existing default transition changes the semantics of Stateflow chart and so the behaviour of the model. This should not be done during verification and validation phase.
However, if you are writing or editing the model, you may use junctions/labels to control entry of default transition. Please refer to the section ‘Label Default Transition’ inMathWorks documentation on default transitionsfor more information and detailed example.