What is a "workstream"?

Beyond that, let it go. It's obvious those tasks are not being shifted to you at this point in time. Just do the work which is clearly yours, and let your management know that you're ready for additional responsibilities. It's up to management when to give you those responsibilities; prodding to acquire work currently assigned to someone else, no matter the backstory, will make you look politically clumsy - you'll be given the work when management decides it's time. It's ok to ask once, but beyond that you're starting to be a pest.

I've been in my role for two years and own a few different workstreams including a big, clunky work stream that I was made the head of but have never really been the head of. When I joined I was told to focus on the other workstreams and the individual who historically had owned this work stream would manage it until I stood up other stuff. Fast forward 9 months, my boss says that I should take over this work stream now but because I'm pregnant and this other individual is shifting roles at the company my boss says it doesn't make sense for me to take it over at this point and so I keep helping with the work but not managing it. I go on maternity leave and before I go my boss tells me that this person will be managing this workstream in my absence and that happens and then I come back and have some awkward conversations with my boss and this person about whether I should take it over since it is in my job description and I feel like that's the right thing to do to take it over. My boss says it doesn't really matter and that person deflects whenever I try to talk to them about it and now I'm trying to just do what I can but that person is running around calling themself the head of this work stream (I don't really care about that too much, but it would be nice if we could just get on the same page and they won't have a direct conversation with me about this) and not copying me on emails. When I ask to me copied the bcc me on the emails, which is weird. This person doesn't report to me boss and they have the same title as me but they have been at the organization for a long time and are well liked, including by me. I'm sort of at loss as to what to do here. I'm really struggling with all of the non-answers that I'm getting about this. I don't really care that much about this work stream because I own other stuff but I do want to grow professionally at my company and get promoted and I fear that losing this work stream could hinder that. Should I just ask my boss about whether not owning this work stream will hinder my growth? Do I try to claw this back? What do I do?