Anker 757 loses AC output

I have recently bought the 757 to use as an U.P.S.

We had a power outage today and the 757 worked as it should continuing to provide A.C. output until the A.C. input was restored. The moment the input was restored the 767 switched its A.C. output off. There were only a few devices connected Modem/Network switch/USB Hub/ Monitor in standby and P.C. powered down.
Power saving mode is set to off.
Is this how it is supposed to function, totally useless if the P.C. was switched on.