This is a perfect example that I've often seen in practice. There's nothing blocking in this workflow at all, and no reason these changes cannot be made in independent changes. e.g.
1) API implementation - Including tests and docs this should be perfectly acceptable to merge and review independently 2) UX implementation - Feature flagged, dummy API responses, easy to merge + review 3) One quick "glue" PR where the feature can be integration tested etc
This prevents awful merge conflicts, multiple rounds of increasingly complex stacked reviews, and a host of other annoyances.
Is there any reason that the stacked PR workflow is better that I'm ignoring or overlooking?