I've noticed a bug on multiple instances in our environment. Syncing with a SharePoint site, using OneDrive build 18.091.0506.0007 and running on Windows 10 Enterprise b1709 and b1802.

With Files On-Demand enabled, and when navigating to any folder in the site, it automatically downloads the files in that folder even though none were set to 'Always Keep on this Device'. Also, when right clicking these files and clicking 'Free Up Space', it switches back to the cloud (online only) and one second later, re-downloads them.

Tested on a newly imaged machine with no other software on it. Windows Defender isn't actively scanning and I've excluded this folder to test if it's interfering.