Hello Shang
This behaviour is normal. Not only that, it is actually desired. The whole purpose of the MPLS Layer 3 VPN is to be able to create a transparent MPLS VPN between the two CE routers as if they are directly connected to each other. You’ve configured the Loopback to participate in the RIP routing, but you’ve also configured the ip vrf forwarding CUSTOMER VRF on the customer facing Fa0/0 interface and the mpls ldp autoconfig on the network facing interface. This will cause all additional network interfaces to be “inside the VPN” and not visible to the RIP process.
I hope this has been helpful!
Laz