simp config currently adds classes to the classes array in the SIMP server's hiera YAML file. It should be modified to look for simp::server::classes array or classes array. It should also abide by whatever indentation is used for arrays in this file. The simp-core default suite acceptance test has a workaround for the fact that the SIMP server YAML file generated from Hash.to_yaml has different indentation.
I've got the changes implemented and am testing them on a 6.4.0 VM.
This needed to be classes or simp::classes instead of simp::server::classes since the items are in the global configuration and meant to apply to all hosts.