Don't use the quick save at all! Use the normal saves, and it should be okay! Are you guys planning on testing that as well?

ie, 15 turns of doing nothing, versus
15 turns of saving and reloading using normal saves?

Maybe quicksave was MEANT not to save the army status (standing or laying siege) when on the campaign map, therefore you lose the siege, but you gain the benefit of quicksaving and loading.