Comment # 1 on bug 1091780 from Wolfgang Bauer

Indeed, I can confirm that here.
It's related to the new feature to support hidden files on NTFS partitions.
Apparently the root directory always has the "hidden" attribute, therefore it
is treated as hidden folder now...

Also affects the file dialog btw, not just dolphin.

There is an upstream bug report already though:
https://bugs.kde.org/show_bug.cgi?id=392913