Haven't tried, but in NTFS, "deallocated space" and "metadata" is not the same thing. Try to recover a file: is it recoverable, or is it garbage? (I would agree that not deleting the name of a file is still a security risk, and freed files metadata should be considered deallocated space, but maybe Microsoft thinks differently).
Or maybe (please do not be offended, it's a thing that happens to everybody: it happened to me) you copied the example from the docs, and
/W:C:\TEST will only erase the unallocated space hanging on the "TEST" node, not the entire drive C: (that would be
cipher /W:C:\). Even if the TEST directory does not exist, cipher will still look for it for a long time, given the impression of being doing something while actually it is doing absolutely nothing.
SDelete or the Tools section in Piriform's CCleaner instead of
cipher /w. Just in case.