@Audumbar kamble 

that's expected. try this

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading) {
        return;
    }

    // Define the states that require clearing the "Assigned to" field
    var clearStates = ['6', '9', '2'];

    // Check if the new state is one of the states that require clearing the "Assigned to" field
    if (clearStates.includes(newValue)) {
        g_form.clearValue('assigned_to');
    } else {
        // If the state is not one of the clear states, retain the old value
        var oldAssignedTo = g_form.getValue('assigned_to');
        g_form.setValue('assigned_to', oldAssignedTo);
    }
}

If my response helped please mark it correct and close the thread so that it benefits future readers.