Zigbee sensors and loss of coordinator

Just starting to add Zigbee to my Home at first just using a couple of Sonoff temperature sensors.

All works great once connected all the time bridge is powered up I see the sensors okay. Last night though I accidentally turned off the bridge, realised and turned it back on. When I looked this morning I realised the sensors were no longer connected.

Eventually I got them connected by pressing the button on the sensors and they started working again. Is this normal behaviour? Do you really have to reconnect all sensors after a power outage or is this a wrinkle of the way Sonoff sensors or maybe my setup works. If it is I may have to have a rethink as constantly going round the house reconnecting every zigbee device would seem unviable.