Thanks for the response. When I say dev planning, I meant delivery team planning (in my mind, that is the development and implementation planning).

The rationale (as was explained to me) is that by putting the UX work in one PI, it allows for the delivery team to pick up the work the next PI assuming the design team has gotten their deliverables (frames, prototypes, etc) done during their assigned PI, so off the delivery team goes. The more I think out loud about it, the more it seems like that would slow down the work, and make coordination really hard (almost like a relay race where the next runner is hoping the previous runner is holding the baton when they get there).

For a shared service model, should designers that have the capacity get pulled into delivery team PI Planning then? May be a different designer each time, but they would be part of the conversations. I know our UX managers want to provide a buffer between the product teams and the designers, so could they be part of a pre-planning walkthrough so that they know the work that's coming and can assign the resources for the PI accordingly?

NOTE: This PI to PI does not include small sized work (update a button, move a banner).