Normal.
This is the overflow port, a drain pipe intended to limit the liquid level being too high.
The overflow port of the solar water heater plays three roles:
First, you can know whether the water is full from the port overflow - looking at the return pipe.
Second, drain the water that exceeds the limit to prevent excessive pressure from swelling and damaging the water tank.
Third, balance the pressure of the water tank to prevent the inner and outer teeth from being unbalanced and causing collapse when water is released.
Therefore, do not block the overflow just because it overflows, otherwise it will destroy the solar energy.