If there is more than one option in the Page Location drop down box when create a new page, when you preview, all other options are removed and you can only select your original selection.

The problem exists on Ikiwiki's wiki, but manifests itself differently, in that subpages are still displayed, so maybe that's the actual bug :)

Anyway, to reproduce, edit any page, view the page location options, preview, and some will disappear.