What version of Windows are you using?
On Windows 10, it seems to behave consistently. I can clearly see when the ".." is highlighted, when dragging the files over the panel, so I can tell if the files go to parent folder or the current folder.

I can also drop the files to the actual file listing on any columns except for the "Name" column, as long as I have the default configuration with "Full row select" turned off.

In what version do you think that it was it possible to drop files on the status bar?

If you copy files into the right side by dragging them from the left side and dropping them on the header by (on say the 'size' header), works most of the time. However occasionally after the directory is full on the right, the copy will actually go to the parent directory even though the files were dropped on the header, not the ".." row.

Anyway, the only reason I'm dropping into the header is because if the right side is totally full, there's literally no good drop target and copying files becomes annoying. The header bar is the only solution I've found. In previous versions you could drop them into a status bar which consistently worked and didn't exhibit this problem.