netflow registering traffic on incorrect interface

I have a cisco 2911 router v15.3(1)T with 2 interfaces on which I want to track Netflow data. One is a tunnel interface and the other is gigabitethernet. The tunnel is a backup path and should always have zero production traffic traversing it when the gigabitethernet interface is up.

Functionally the router is doing what it is supposed to. All the traffic is being routed through the gigabitethernet interface. when I do a "show int Tunnel1701" I get the following output, which appears correct:

-----------------------------------------------------------------
  Output queue: 0/0 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     1556 packets input, 137192 bytes, 0 no buffer
     3 packets output, 408 bytes, 0 underruns
-----------------------------------------------------------------

However, the associated sensor in PRTG is registering between 200 and 1000kbps of traffic continuously. Here's the relevant config on the router...

-----------------------------------------------------------------
 ip flow-cache timeout active 1

interface Loopback0
 ip address 192.168.96.5 255.255.255.255

interface Tunnel1701
 description DMVPN backup WAN interface
 ip flow ingress
 snmp ifindex persist

interface GigabitEthernet0/0
 description WAN interface
 ip flow ingress
 snmp ifindex persist

ip flow-export source Loopback0
ip flow-export version 9
ip flow-export interface-names
ip flow-export destination 192.168.1.187 9995
-----------------------------------------------------------------

And the relevant interface indices are 2 and 20:

-----------------------------------------------------------------
SP-RT-CS2911-01#sh snmp mib ifmib ifindex
GigabitEthernet0/0: Ifindex = 2
Tunnel1701: Ifindex = 20
-----------------------------------------------------------------

In PRTG I have these sensors defined as Custom Netflow v9. The receive port is set to 9995, the Active Flow Timeout is set to 1 minute, and sampling mode is off. All my custom channels are based on only source and destination port number, nothing else. Logging of stream data is disabled. The include filter is "Interface[20]" for the tunnel and "Interface[2]" for the gige port.

Am I missing something important? Oddly, the PRTG graphs for the two interfaces are NOT identical... but the tunnel graph shows continuous traffic in the hundreds of kbps which the routing table and the "sh int" output of the router indicate does not exist.

THanks, -Mat Rouch

cisco filter netflow