Allies not protecting territories

Yesterday, I was helping an ally protect some territories against an assault by a mutual enemy.

On territory had a level 2 fortification, and 4 of my troops. The enemy attacked with one unit (old order)... and he captured the territory and I had to fight him with him getting the fortification bonus.

A second occurrence happened an hour later where I had troops defending another territory (at least no forts). It immediately switched side.

A territory center needs to be clear of all enemy troops before it can be captured.

Ham