That was happening to me after upgrading my hardware (& current at the time SW). Rob gave me new (beta) SW to try and it hasn't happened since. But I don't think that's the production SW yet.
A workaround Rob also suggested was to leave some other item that draws power on (light, fan, whatever) when you shut down the master / EFIS. Something about where the bus voltage level is at the moment it starts the countdown. Once the countdown has started you can shut down whatever else you left on.