Today the ACME is working one by one in the order of the list. I wanted it to work in round robin so it will call to different AMCE every time so it will reduce the chance for block.
CertMagic recently added support for “issuer policies” – i.e. the ability to customize how an issuer is chosen; right now there’s “First” and “First Random” – there’s no round robin yet because that requires state and that’s more complicated, and there’s little evidence that it would be objectively more suitable than random in most cases.
I don’t think I’ve exposed these options in Caddy yet. I’m a little busy right now but it’s something that I could prioritize with a sufficient sponsorship.