We should also mention that there’s a popular smart app which does basically the opposite, once the power comes back on, it turns all the smart bulbs off. This is because they’re assuming that the bulbs were off during the day anyway, and they don’t want them coming on in the middle of the night and waking everyone up if there was just a temporary power outage.
You don’t need this kind of smartapp for a smart switch, because those typically remain in the off position when the power comes back on.
But smartbulbs, particularly zigbee smart bulbs, do tend to come on full bright when the power is restored. This is an intentional design, and is intended to handle the situation where someone has manually flipped the wall switch. They will expect the lights to come on when the switch is flipped back on.
But as we mentioned, there are community members who don’t like that feature, and so one community member did write a smartapp that just turns off all the smart bulbs when the power is restored.
So as always, different things work for different households. 