Pre-upgrade validation for PostgreSQL Flexible Server (v11 → v16) returns all checks as "Failed" with no error message

When running the in-place major version upgrade pre-validation (PG 11 → PG 16) on my Flexible Server, all 16 checks return "Failed" with an empty error message (including trivial ones like "no prepared transactions" / "no event triggers"). I've already ruled out the documented causes: password_encryption set to scram-sha-256 and admin role regenerated to SCRAM; pending_restart query empty; no user event triggers; only extensions are plpgsql, pg_buffercache, pg_stat_statements, unaccent (all supported). It looks like the pre-check isn't actually executing the individual checks. Is this a known issue, or how can it be diagnosed?