Have you opened your folder with the media files, that it gets a chance to generated the preview images. I am not sure if the media overview starts generating these. If you want to automatize that in the background you can use the preview generator app: Preview Generator - Apps - App Store - Nextcloud and run it as a system cronjob.
I’m facing a similar problem with NC 27 (official Docker). Preview generation seems to work fine with all document types except PDF files… I can’t see any errors (even in Debug mode) in the logs.
Not sure if this is related to this issue here, but wanted to share that.