According to below test schematics. When MCU send ship mode CMD to MAX77734 through I2C, MAX77734 open the Switch betweeen Vbatt and Vsys, so the Vsys will drop to "0", then the Vbuck drop to "0", so the nEnLdo voltage is about Vbatt*4.7k/(200k+4.7k), (Vccint =Vbatt), the nEnLdo voltage is less than the nEnLdo falling threshold voltage, so the MAX77734 judged that onkey is active, then MAX77734 exit the ship mode and re-power on.
Adding a Diode between MCU's GPIO_turnon and MAX77734's nEnLdo, the issue was fixed.