EAMxx: requesting a diagnostic field inside a diagnostic

The idea of our diagnostics is that we can query them inside each other, and we can do composable and fancy stuff online. It took me a while to realize this wasn't the case, and the diagnostic field dz was simply always zero. It's a bit bizarre, because clearly it is there as add_field("dz"...) didn't error, and get_field_in("dz") didn't error.

There might be something I am missing or something I am assuming... but there's a larger chance that we have bug :)