It was caused by faulty triggers for traits. I think it was 'enemies in battle' trigger or something. If the trigger for a trait is complex and the trigger includes counting units on the field, it would crash when you loaded out of the battle. Since it was only confused when multiple armies were on the field, it crashed during battles with reenforcements.
The bug still happens, very rarely. But in 7.x, it would happen about 50% of the time when there was 3 or more armies fighting.
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