It looks like no amount of refreshing or forcing results in downloaded subtitles. I looked at the logs and I'm seeing this:
2018-01-07 20:28:52,948 (7f55ae16e700) : ERROR (model:205) - Cannot read model from /volume1/Plex/Library/Application Support/Plex Media Server/Metadata/TV Shows/0/dd872be2dd02a1b21cf1da010e9d8e74490092a.bundle/Contents/com.plexapp.agents.subzero
I just looked at that folder, and the Plex user does have Full Control.
I'm not sure where to upload my logs, but they're here if needed.