Do you play with battle timer on ?

[Didz]

@Savage Smurf
If I am reading your post correctly, then you made a big mistake.

If by 'I decided that I should bring the fight to them' you mean that you decided to launch a sally attack on their beseiging army, then you could not have lost that battle.

Had you chosen instead the Quit and Exit the battle it would have been declared a Draw and you would have kept your city. The only way to lose a sally battle is to allow all your troops to rout.

The passive assault bug only really has an impact if the battle is started by the AI.