We’re always here to listen! Part of the hard part of being UXers is that we are very often smaller teams - often teams of just one or two people. It makes finding a community and support network in other places extremely important. That’s precisely the reason I’m here so friggin’ much.
Personally, I’m always happy to listen and help how I can.
I’ve had this happen before, and like any designer it ground my gears. We’re often so close to our work that it’s hard not to take changes like this personally.
What’s even harder is to think about the problem academically in the moment. The hardest part is asking yourself if there a compelling business or design reason for moving the buttons that you might have missed.
My advice, if this happens again, is to raise your concerns in the moment. As you say, you are responsible for the whole product. You weren’t given a chance to raise your concerns in the moment, but that doesn’t mean you shouldn’t raise them now. You are not just speaking for yourself, you are speaking for your product’s user base.
Take a quick, deep breath, raise your hand, make the observation that the buttons were moved and provide your reasoning for their original placement, which should be user-focused and supported by research or best design practices.
I would also take a moment, privately, to speak with your boss about your concerns. Again, this conversation shouldn’t be adversarial, but you do need to stand up for your work. Explain your thinking once more, emphasize that you take your duty to the product seriously, and ask to be consulted or at least made aware of design changes in the future so that you have a chance to make appropriate objections in the moment.