Media reference field gets target_id set to 1 for every row

Field values for new media entity references on node bundles are not getting populated

Here is the migration config I'm using:

  field_image_media:    
    -    
      plugin: sub_process    
      source: field_thumbnail    
      process:    
        target_id:    
          -    
            plugin: migration_lookup    
            source: fid    
            migration: upgrade_d7_file    
            no_stub: true    
          -    
            plugin: update_file_to_image    

When using the update_file_to_document plugin, this config works for creating new media entities and populating the node entity reference fields with the correct values. For image media, the field_image_media_target_id (for a field_image_media field) is always 1, which is obviously not correct.

I've gone over this a little in the #migration slack channel, and was happy to get the migration working for document (file) fields, but I haven't been able to get the image fields to migrate in the same way, which is why I'm opening an issue here, for some more visibility while I continue to debug my process.