Filed upstream as flutter/flutter#187611.
Minimal reproduction: on iOS, selecting a Material DropdownMenu entry wipes the
entire UIKit accessibility tree — but only when the app uses go_router /
MaterialApp.router (the Router API). A plain MaterialApp with the identical page
is unaffected.
Flip the useGoRouter flag at the top of lib/main.dart to compare
the two cases.
See ROOT_CAUSE.md for the full writeup: environment, steps, expected-vs-actual, the isolation matrix (what was ruled in/out), and the relationship to flutter#186582 / #187167.