I thought I would update this in case anyone else has a similar problem. Since the first post, I took out the new module 42, and it too has the same problem (4V one side, 0V the other side). It had also expanded. So the car is damaging the batteries.

I think the problem is the voltage measurement wires between the battery and the BMS. I found a short between the monitoring terminal on module 42 and the positive terminal on module 43. This makes me think perhaps module 42 is being overcharged when the charger is connected. I had to extend these monitoring wires when I built the car, so perhaps now a few years later this short has happened and is causing the problem.

I haven't totally fixed it yet, but I think this might be the key. I have got the extended wires out and when time allows I will search for the short then rebuild the extended wires.