This is a known bug with the esp8266 sdk. I ticket has been submitted. The problem mostly occurs when there is a power outage and your Sonoff devices boot up before wifi is available. If you have a lot of power bumps it may be beneficial to put your AP on a battery backup.
I have dozens of esp devices, but rarely have seen this problem in the wild (because I don’t have very many power issues and my AP is on battery backup). I can easily replicate it though.