On my Insider build (currently 17040, but the issue is clearly much older) I have Windows.old, windows.old(1), ... Windows.old(4). System disk cleanup has been done (and I do it soon after each new build as I don't have much disk space). Space is reclaimed on cleanup as expected. All these Windows.old folders are empty, or at least appear so.
If I attempt to manually delete any of them I get a popup saying "This folder is no longer on C: drive....."
What's going on with these folders? I apparently have adequate permission to manage them. Any ideas?