Large request hangs

After migrating a service into a new VPC behind a VPN/overlay tunnel, small requests work perfectly but any request with a large body (file uploads, big JSON payloads) hangs and eventually times out. curl with small payloads returns instantly; the same endpoint with a 20KB body never completes. Packet captures show the large request's first few packets arrive, then silence, the connection stalls mid-transfer and no RST is seen. Pings of normal size succeed; ping -s 1472 -M do (don't-fragment) to the peer fails with no reply. The overlay/VPN was introduced in the migration. Diagnose and fix.