hundredmph farmer
A couple of possible ideas/
Anything plugged into your accessory ports? Something on the accessory circuit might be keeping the ZFE alive after key off.
Anything like a GPS plugged into the little GPS plug at the front of steering neck? If so try disconnecting that as a GPS coming back on after key off could be reinitiating the ZFE controlled accessory circuit.
Is your battery old? If so possibly have that load tested.