I am sure the Mitsubishi dealer would know more than me, and I hope all is sorted soon.
I had a think about the problem and wondered, even though it seems unlikely, perhaps the BMS for some reason senses the battery is full, and is doing what I have seen the car do, in that when I wanted to be very sure the car was full for a long range test, I charged the car normally, then after it finished, I replugged it in and charged again, and the fan went on briefly but the charging light went out in a few minutes.
I did a turn off and replug in yet again, and the same thing happened, the fan went for a bit then the charging light went out.
It is unlikely it would work to do anything if there is a fault, but I wonder if a reset of some kind might work, if you disconnected the 12 volt battery for 30 minutes, preferably after making the main car battery to about 3 bars or less.
Then reconnect the 12 volt battery and then try a recharge, it may work or it may not, but perhaps worth a try if nothing else does.