When I do the initial file migration from my old Drupal 7 site, my source file paths are respected and maintained when copied to my new fresh Drupal 8 site. In other words, inside the 'sites/default/files' directory on the new site, the same hierarchy is copied over from the old site 'files' folder.
However, when I go to migrate content types with file fields and image fields in them, the file path is changed to use a year-month sub directory pattern. I removed this default pattern from the file/image fields definition, yet it continues to happen in my tests.
I would like to have the fields migrated and maintain the old structure. So basically attach the already migrated fid to the file/image field without moving files around on the server.
I just noticed that one of my content types has the file field retaining the proper path on running the migration while another has the behavior described above. I double checked the file field settings in each to make sure the path is set to empty in the config.