I'll add here that there's a ton of nuance to "pushing back on changing requirements." My rule of thumb is this: program managers should understand the expected and worst-case consequences (including baseline time, switching costs, stepping on other team members' toes, etc.) of any proposals to expand scope, and as an individual contributor it is vitally important to be vocal to ensure they understand the full dynamics.

But if and once they do understand and acknowledge the consequences, and still say a scope increase is justified, it's highly highly likely that they have more business context than you. Document the decision, of course, but fundamentally trust your team.