[FastBoot only] Error: No model was found for '@ember-data/model/-private' and no schema handles the type

[lolmaus]

Hi! During a routine upgrade of Ember CLI, ran into #338, which required bumping ember-data-model-fragments to 5.0.0-beta.0. This lead to the error in title.

I see it only in FastBoot after upgrading from Ember 3.12. Happens both on 3.16.something and 3.19.0.

I cannot debug it in FastBoot.

The full stacktrace led me to these lines in Ember (!) codebase:
https://github.com/emberjs/ember.js/blob/master/packages/@ember/-internals/extension-support/lib/data_adapter.js#L354-L356

Our app is quite large and creating a minimal reproduction is not feasible.