yaFyaml does not fully support yaml. It can not parse all of yaml.
Also @snano, You should use fortran-yaml-c instead of fortran-yaml-cpp. Long story short, Fortran and C are more interoperable than Fortran and C++.
It is possible to output yaml with fortran-yaml-c (or fortran-yaml-cpp - the API is almost the same). but it is a bit clumsy. Here is an example