Honestly, I think the thing you should be asking is: Why didn't you get promoted to senior staff engineer and given leadership over the two stacks, but rather t

Honestly, I think the thing you should be asking is: Why didn't you get promoted to senior staff engineer and given leadership over the two stacks, but rather the other person did. If it's a team that needed help, rather they give it to the more senior and experienced person, and you get more experience in your new role whilst managing an existing team that is proven and you have domain-experience over. To me, what this says, is that the other senior staff engineer was given a "promotion" in the form of managing "two" stacks, i.e. bigger head count. And you were essentially demoted to being moved over to another stack and team. Doubly so if that other team or stack or project is not seen as that important. The title they gave you was a way to placate you about the effective demotion. But even that interpretation could be wrong. At the end of the day, it's the machinations of the company and based on decisions made by people in the "right room", a room you weren't a part of. No amount of rationalizations will make your feelings of the topic go away. I know this because I've been in similar situations, and those feelings never really go away, you will always feel slighted. Even if you raise it with management, at best they'll make you go away with manager-speak, at worst it'll colour every negative or hiccup that happens in your new position or project.