I can confirm this behavior. I'm not sure, whether this qualifies
as implementation or a documentation bug, though. Of course, the
mixed directory separators are usually not desired, but still the
implementation would make some sense, as only the given $path will
contain Windows directory separators, but not the rest, cf.

    <?php
    $files = new RecursiveIteratorIterator(
        new RecursiveDirectoryIterator(
            __DIR__,
            RecursiveDirectoryIterator::SKIP_DOTS | RecursiveDirectoryIterator::UNIX_PATHS
        )
    );
    foreach ($files as $file) {
        var_dump($file->getPathname()); // paths with mixed dir separators
    }

> I often find myself wondering why backslashes are used in paths
> on Windows at all?

Consider exec(), for instance.

> Of course, the mixed directory separators are usually not desired

I think it's fair to say, they're never desired - you want either UNIX or Windows paths, there's no scenario (least of all printing to display) where having a mix isn't just "weird".

> but still the
> implementation would make some sense, as only the given $path will
> contain Windows directory separators, but not the rest, cf.

I don't think that makes sense. You'd have to update the documentation to explain that it only normalizes the sub-paths - but that would be completely pointless; if I have to normalize the base path myself, that means I have to normalize paths myself, which means I might as well just normalize all the paths myself. As is, this feature suggests I wouldn't need to, but that's not true.

What possible use would anyone have for a partially normalized path?

I don't see how this is not a bug.