The unit waits several seconds after power is restored, to see if it settles at a stable voltage. (Your unit is designed to detect 380V from the grid, and stay connected as long as it senses that voltage.) An internal relay switches your home back to grid power. However your 220V may not be high enough to cause it to decide it is time to switch to grid power. Or, in the case of the relay, 220V may not be sufficient to activate a relay designed to activate at 380v.
If instructions don't specify how it responds then you'll need to run your own tests.