The readme says:
As mentioned above you need to add the {{head-layout}} component once and only once in an application wide template
But I'm finding that I need to include {{head-layout}} at whichever route level I interact with the headData service in order for it to work with FastBoot.
For example, if I have /profiles/1 which corresponds to an application route and a profile show route, both of which set data on the headData service, then I will need {{head-layout}} in application.hbs as well as profiles/show.hbs in order for the changes in the profile show route to be reflected after FastBoot render.
Hope that makes sense.