As per datasheet, the soft start capacitor will discharge if the SHDN pin voltage goes below 0.6V. Connecting the SHDN pin directly to the IN Pin will help to solve the problem. The MAX8869 works as follows:
with the soft start capacitor, the part will start with a reduced current limit. If this reduced current limit is enough to charge the output capacitor [with Load], we will not able to see the soft start of the output voltage on the scope. If the output capacitor is high, the part will start with the reduced current limit and we can see the soft start of the output voltage on the scope.