There is a a bug in Plex old localmedia agent for python based agents, it will not search for any subtitles if the season folder is year based, for more information please take look at this plex bug report.
To fix the issue at least for linuxserver/plex containers. do the following, this has to happen everytime the container restarts as plex somehow redownload everytime the container restarts.
first gain shell access to your docker container by doing docker exec -ti container_name bash change container_name to whatever you named your container to be. Then execute the following commands.
$ sed -i 's/int(s) < 1900/int(s) < 3000/g' /usr/lib/plexmediaserver/Resources/Plug-ins-*/LocalMedia.bundle/Contents/Code/__init__.py
$ kill -9 $(pgrep -f 'com\.plexapp\.agents\.localmedia')