The problem with what you describe is very common. The team is receiving requirements, not creating them collaboratively.
Agile principles:
Business people and developers must work together daily throughout the project.
The best architectures, requirements, and designs emerge from self-organizing teams.
In the common scenario you describe, the Product Owner and UX have decided that the team doesn't need to talk with stakeholders, and doesn't need to be involved with writing requirements.
There's no one-size-fits-all answer. Sometimes they will come up with decent requirements. The problem is that if they're not technical they may only see a certain range of solutions to the client's problems. (UX designers have a tendency to look for solutions that involve a UI.)
Other times they'll come up with requirements that don't make sense from a technical perspective.
In either case, they're likely to have meetings where they tell the rest of the team
This is your opportunity to look at the design and ask questions.
That's not collaboration. That's a nice way of asking
Do you understand what we've decided that you're going to do.
Again, I can't propose a one-size-fits-all solution. I'd recommend something more like this:
- Everyone working on a product has some degree of contact with users and stakeholders, even if it's occasional meetings. Developers should be able to straight to users with questions. If there's concern that developers will say the wrong thing and make promises, well, don't do that.
- When the team understands what problems the user needs to solve, they discuss possible software solutions. Again, stakeholders and users should be present.
- When everyone agrees on a likely solution, they design it together. If you have a UX expert on the team they can contribute a lot to this. If you want to sketch something out together and let the UX designer create a polished version, go for it.
- Build the product.
- If the UX designer thinks something should be two pixels to the left, they can use their hands to go into the code and change it.
UX designers are valuable, some more than others. They must act as part of the team. They are not above it or outside it telling it what to do. Neither Product nor UX should interfere with the communication between the team, users, and stakeholders.
I'll allow for one big exception: If you're Amazon or Pizza Hut this might not hold up. Maybe you need an uber-UX-expert who monitors peoples' brainwaves as they look at different designs. Maybe the choice between A and B means millions of dollars. In that scenario I understand if a company prefers to do more up-front design.
That's the exception, and even then UX shouldn't go too far off the rails before consulting with the rest of the team.