no active trail for parent pages in menu block that uses book navigation

I read through #1017142: except for one menu, all other menus (including books) never receive an active trail but I still seem to have the mentioned problem (no active trail for parent pages in menu block that uses book navigation).
E.g. see the "Trading Carbon" block on the left here http://www.fern.org/book/trading-carbon/case-study-us-sulphur-trading-sc...
I've done a dsm in menu_tree_add_active_path(&$tree) and the $tree_with_trail claims "Carbon Trading", "Chapters", and "The origins of carbon trading" are not in_active_trail if that's of any help.
Any ideas? I've also tried disabling all other (core) menu blocks on this page but no joy either..