Take the entire Grim Dawn folder that is currently in the OneDrive location, cut it out, and move it to C:\Users\USERNAME\Documents\My Games
If you are sure OneDrive is completely disabled, or uninstalled, I would suggest also rebooting the computer to help ensure it ‘sticks’. Then perhaps check the permissions again on the root Grim Dawn folder you moved.
Hopefully after that it will begin behaving like intended.
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As I suspected from the start, it was OneDrive from the beginning. As it so often is.