How can i debug why files are not being created when using migrate files to media module?
You have an an error in your yml file, should be:
# Map the field
field_media_image/title: title
field_media_image/target_id:
    plugin: media_file_copy
    move: FALSE
    reuse: TRUE
    path: 'public://grants/'
    source: file_path

langcode: langcode