I would bring it up with your manager regardless. This director sounds like he's only looking to further his image in the industry by taking credit for everything possible (but definitely dont bring that up with the manager.)
I would suggest to the manager that by you receiving sole credit for the Written By, he (the manager) is bettering the careers of two clients instead of one.
The decision is up to you. You can fight for your right and potentially upset the apple cart or bury your head in the sand but potentially gain greater bargaining power on your next script. Remember that 'produced writer' is 10000 x times better than being unproduced.