It looks like there is another new OneDrive’s feature on its way to Windows 10, automatically turning locally cached OneDrive files to be cloud-ready when not being accessed for a certain period of time.
The feature is introduced as part of Storage sense feature to save local storage space. It’s enabled by defaut and set to 30 days. So if you are subscribe to Windows 10 Inside builds, you may see your locally saved OneDrive files becoming on-demand with a cloud icon after 30 days.
To configure this new feature, go to Settings > System > Storage and click Change how we free up space automatically.
Scroll down a bit until you see a section called File On-Demand, where you can change the day from Never up to 60 days. Choose Never if you would like your local saved files remain the same.
The feature is first appeared in Windows 10 Inside build 17692, a beta release for the next feature update scheduled to be released in Fall 2018.