echo-sockmap and delays in ebpf setup

[DuratarskeyK]

I found this example for ebpf sockmap and tried to write my own thing, but multi-threaded, though it's irrelevant, since the problem is not there. When adding a socket to sockmap that already had data sent to it, it doesn't work. You can see it for yourself with your example, if you add sleep(2) here:
https://github.com/cloudflare/cloudflare-blog/blob/master/2019-02-tcp-splice/echo-sockmap.c#L132
And then do echo Hello | nc wherever-echo-server-is-listening port
I wonder if there is a simple way to fix this, but so far I couldn't find it, I'm hoping you can help, as I don't even know where else to ask about eBPF issues.