I'm going to take a guess and say that you might have got an adapter wired up for RC cars, which will be reversed.
I've had them myself, but caught it beforehand luckily, and seen plenty of people caught out by this.
IIRC, the larger sized contact on the deans should go to the round contact on the mini Tamiya.
Can you post a photo to confirm?
Also, put the affected batteries outside somewhere safe!
I believe the correct protocol is to cut off the contacts (one wire at a time) and then put it in a bucket of saltwater, but others can confirm.