Exchange SE HU6: PDF attachments truncated to 13 KB via Outlook Desktop — OWA unaffected

We've spent days isolating this and ruled out everything we could touch. The corruption survives agent disabling, Bitdefender removal, and BypassFiltering — and the message tracking logs show exactly where it happens.

Environment: Exchange Server SE, Build 15.2.2562.41 (HU6 / KB5081755), Windows Server 2025

Problem: PDF attachments sent internally via Outlook Desktop (MAPI) arrive corrupted at ~13 KB (original: ~32 KB, no xref/EOF). All PDF sizes, all internal recipients affected. Started 21 May 2026.

Key finding — OWA works, Outlook Desktop doesn't: Sending the identical email via OWA → attachment arrives intact. Outlook Desktop → truncated.

Message tracking proof: Both paths deliver the message at full size (~42 KB) via STOREDRIVER DELIVER. Only the Outlook Desktop delivery shows an additional X-SDDS=0.106 step in the STOREDRIVER latency breakdown. That step does not appear in the OWA delivery. The corruption happens inside that MAPI/TNEF store write step — not in transport.

Systematically ruled out:

• All transport agents disabled → still 13 KB
• Exchange Malware Agent + Set-MalwareFilteringServer -BypassFiltering $true → still 13 KB
• Bitdefender GravityZone fully uninstalled from server → still 13 KB
• EEMS mitigations: only PING1 and M2.1.0 applied, neither affects MAPI delivery

Temporal correlation: Three Windows updates installed 21.05.2026: KB5087051 (.NET Framework 4.8.1), KB5087539 (Windows Server 2025 CU), KB5089717 (Servicing Stack). Exchange SE HU6 (KB5081755) was installed around the same period.

Workaround: Sending via OWA works. Not acceptable long-term.

Has anyone seen this? Is this a known regression in HU6 or KB5087051?