I have a saved game where I get attacked by Ceresa at the end of the turn in one of my cities.  This city has 6 units in it, but not all of these units take part in the ensuing battle, in particular always my mage unit.  I thought it might be because there is a maximum number of units that can enter a battle, but no matter what I do, I can't get the mage to take place in the battle.  I tried moving three of the units out of the city prior to the battle taking place, but the mage still doesn't partake.  After the battle is over, the mage unit is just standing in a square next to the captured city.

https://www.dropbox.com/s/12x21hl4vj01jqr/Campaign3.EleSav

Any ideas why this is occurring?