Hi @Neilpang,
I have checked some of our logs and it appears only one of the validation servers are able to fetch the challenge response when trying to issue using acme.sh in standalone against staging. Upon a quick look in acme.sh, it appears the netcat closes after sending off a response to one validation server.
With the “multi-va” approach we have added in staging, a client will receive multiple validation requests and be expected to respond to each.
Hopefully that is helpful.