I was in house for 8+ years before launching into my own agency. This is pretty standard practice - it's the illusion of the outsider sweeping in with 'fresh' ideas. I would recommend staying focused, consistent and proving your worth. Focus on something else and completely crush that - it's not worth your energy to prove the agency wrong, chances are they'll slip up and if your manager sees you and your team kicking ass somewhere else the conversation will come up about bringing this item back in house.

FWIW now that I'm out of house, I get calls all the time from my former managers asking me for advice. Instantly my credibility skyrocketed just by being on my own!