A valid usage of ssh config files is includeing a directory of configs:
include ~/.ssh/config.d/*

fast-ssh complains that the config is invalid:

SSH configuration contains the following errors:

* SSH option `Include` provided before `Host` is specified.

Moving the include line to the end of the file then includes it with the last host in the config which is undesired behaviour.

A suggested behaviour could be to treat files in config.d as groups, I use config.d files to keep customer devices out of my main config which keeps the main config cleaner and allows me to share specific customer configs with team mates.