GRE Tunnel Issue

In case someone stumbles across this article in the future: TGWs in AWS utilize ECMP and as of this writing you're unable to turn that off. We were experiencing asymmetric routing as a packet destined to AWS was being sent out one VLAN and returned via a different VLAN. It was visible in the traffic capture but I was hung IP on the ICMP response due to traffic returning on the wrong VLAN where there wasn't a listener. 

All credit to F5 support as they found the issue.