I had this happen to me many times, usually after some spirited highway driving, coming to the dead stop. No idea why but if I had to speculate here is my list of most likely reasons:
1) fussy logic decided that I'm in a rush and need good peformance, keeps the engine running so I can accelerate faster and won't be complaining the car is a dog
2) hot temperature inside electric drive due to boosting car speed. Since starting from dead stop requires most energy, the ICE is running to relieve electric motor from pushing the car alone and let it cool off
3) soaking up the heat, the ICE's head is hot due to fast driving, rest of the engine is much cooler, engine keeps on running to dissipate heat from cylinders and pistons
4) something to do with polution and catalytic converter
The energy meter is just graphic aproximation to what happens with the drive system and it's not 100% accurate. If you had your car in "D" and the car was stopped, MG had to be spinning and generating electricity, there is no way around it. I don't know if there was enough "juice" to actually charge the battery. Sometimes putting the car in "P" will force the ICE to shutt down quicker, also everything goes back to normal if the car sits in traffic for more than few minuttes.