ACS Teams interop meeting chat suddenly returns 403 Forbidden on all sends since late Aug 2026

UPDATE (Aug 31): new finding that isolates this to the ACS interop path. In the SAME meeting where every ACS SDK user gets 403 on chat sends, an anonymous participant joining through the Teams WEB CLIENT (display name only, unverified - no email OTP) can send meeting chat messages normally. So the tenant, meeting options, and organizer policy demonstrably permit unverified anonymous meeting chat - the failure exists only for the ACS client type. BlockedAnonymousJoinClientTypes is empty and all other anonymous-related controls are permissive (verified as Global Admin). Also tested - a brand-new meeting created after assigning a test policy with AnonymousUserAuthenticationMethod=None still fails with 403. Severity-A Azure support case is open.