Webfonts declared in custom styles json files are not enqueued. No error is thrown, but the fonts are not loaded in.

To reproduce:

1. Add a style variation to emptytheme.
2. Add font file(s)
3. Declare the font using fontFace under settings.typography.fontFamilies.
4. Open the Site Editor.
5. Select the variation, notice that the correct font is not loaded.

Example PR here: WordPress/theme-experiments#304

In the PR example, the same code works properly when pasted into theme.json instead of styles/variation.json.

Screenshots:

The correct font for the variation should be DM Sans. Instead, the default sans-serif fallback is being used, and no network call to the DM Sans font files is being made.