At my current work ( a large enterprise org), the dev team and designers run under different managers and different sprints. The sprints are aligned such that the UI team stays 'a head' of the dev team such that the UI specs are available to the dev team for sprint planning.
I don't like this method because it really creates siloed development and it feels like the UX team is simply tossing designs 'over the fence' for us to work on. There is very little communication during the design phase.
On the other-hand, the UX team is responsible for work in addition to what my dev team works on, so I have to respect that (and thus why they run their sprints on their own and not part of the devs).
So I believe the dev team shouldn't dictate designs to UX but I do try and schedule a preview of the designs to the dev team early in the design process, so we can hash out problematic areas in the UX design early on.
I could talk about the issues and solutions for hours...but...its a Saturday night :)
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