When using flat view, it's very easy to accidentally drag&drop a file to current directory (flat view top directory) by moving any file just a little bit.
I can remember you disabling this at some point but now it's back.
I think in the past we didn't support it at all, but support for it was added as not being able to do it made it more difficult to flatten directory structures via drag & drop than it should have been.
Moving to the current directory is quite common and as valid as moving to anywhere else, really.
You could probably change the All Files & Folders drag & drop event to run a bit of script code that checks if Flat View is enabled and if the destination is the current folder, then blocks the operation or asks for confirmation.
That's not the point. When you are not in flat view and you accidentally drag&drop a file a little to the right, it's dropped to the same folder it's currently in so no move operation is performed, so I'm kind of used to that if I don't drop to a specific folder, nothing happens. However, if you are in flat view and move a file just slightly to the right, it's dropped to flat view root folder. It's in my opinion too easy to do that by accident.
I just tried to do the same on 11.19 x86 and drag&dropping to empty area in flat view is not possible.
I also tried on 10.5.7.0 x86 and here -- on the other hand -- it is possible.
So it looks like you had this feature, then disabled it (I'm not 100% sure but I think that you pointed out somewhere in release notes that you disabled it because it's too easy to move something to root by accident) and now enabled it back again.
Anyway, I liked it more when it was not possible to move a file/folder that way, so 11.19's behavior was better for me. If you are so uncertain about this feature, maybe it's a good sign that it needs an option in preferences? 
Of course this refers only to dragging a file/folder within one lister. If I move a file from other lister (other window or the other lister in dual mode) or folder tree, I'm completely fine with drag&dropping to empty area of a lister.