i dont believe sonarr goes searching for things in subfolders so any extra files you want to keep must be in the same folder as the video file is, for it to get imported as well.
ie the import code is told which folder the video file is in, and thats where it looks for it, no where else, same for any extra files. i’d be willing to bet that if someone created a season pack where each episode was in its own folder sonarr would have a similar issue with importing them.
the logical reason for not searching subfolders is being able to handle duplicate filenames - and if it did check for content in subfolders and then created matching subfolders in sonarr those subtitles would be useless until you manually moved them into the parent folder.
basically it gets messy so its kept simple instead