So I wouldn't say C5s don't work at all with NLB instance target groups - I'm running one right now.

I wouldn't be surprised if you're hitting an edge case with the tighter integration* between NLB's and instance associations. If you haven't already, please do reach out to support.

* from the docs "If you specify targets using an instance ID, the source IP addresses of the clients are preserved and provided to your applications. If you specify targets by IP address, the source IP addresses are the private IP addresses of the load balancer nodes."