Where are the movies stored? If it's a hard disc drive maybe it's spinning down after 20 minutes?
You can change this setting in power options.The default is 20 mins, so I think this is your problem: https://www.itsupportguides.com/knowledge-base/windows-10/windows-10-how-to-control-hard-drive-power-down/
I had similar problem years ago, not with jellyfin, but it was on a linux server. On linux you can change this behaviour with hdparm