this report has been filed here:

https://bugs.edge.launchpad.net/ubuntu/+source/nautilus/+bug/588872

"When a folder is open, its icon in the parent folder window (including desktop) changes to show its open status; this applies both to standard folders and built-in special folders (documents, templates, music etc, except desktop).
Custom folder icons, however, are static - they keep the same icon when open [this is similar to what happens to home folder, but affects any folder with custom icon].
The standard behaviour is restored upon restoring the folder's default icon (either standard folder or special folder icon).

Expected behaviour: all open folders should show with standard "open folder" icon, regardless of what custom icon has been set for them.

Possible enhanced behaviour [wishlist]: folder options could feature two icons, for closed and open folder, allowing to choose whether to leave the default open folder icon, to set it to the same custom icon as the closed folder (eg for non-folder-shaped icons) or to customize both icons. Alternatively, there could be a "use 'open folder' icon" (or simply "static") option to switch behaviour."