An engineer connects an on-premises Cisco IOS XE router to a Cisco Catalyst 8000V deployed inside a Google Cloud VPC over an IPsec VTI and runs OSPF across the tunnel to exchange routes with that cloud-hosted router. The IPsec SA is up and the tunnel interface passes ICMP, but no OSPF adjacency forms and 'show ip ospf neighbor' is empty. The engineer confirms both sides use OSPF area 0, matching timers, and the same network type. Reviewing the IOS XE configuration reveals 'passive-interface Tunnel1' is present under the OSPF process. What is the most likely cause of the missing adjacency?