A network engineer configures a GRE tunnel between two branch routers. The tunnel source is set to a loopback interface, and the tunnel destination is the remote loopback. After configuration, the tunnel repeatedly flaps and the logs show '%TUN-5-RECURDOWN: Tunnel0 temporarily disabled due to recursive routing'. The engineer confirms both physical WAN links are up and the loopbacks are reachable via a dynamic routing protocol. What is the most likely cause of this issue?