migration – Migrate media with alternative text

I’m trying to import image with new media module on core 8.5.

I’ve a json source and on my .yml I’ve:

  field_media_image:
    target_id:
      plugin: migration
      migration: immagini
      source: fid

This works, immagini is another importer that imports files.

In the new media there is also an “alt” (mandatory) and “title” attribute on the media entity.
But if I try to add something like this, it doesn’t work (alt_text is the correct source for alternative text). The media is created but is not referenced to the correct id:

  field_media_image:
    alt: alt_text
    target_id:
      plugin: migration
      migration: immagini
      source: fid

How should I pass the alt attribute to this field?
I also tried to use a syntax like:

  field_media_image:
    target_id:
      plugin: migration
      migration: immagini
      source: fid
  'field_media_image/alt': alt_text

But it doesn’t work, the media is created but there is no reference to the correct id.