The "Pages which have been changed since you last visited them are shown in bold" feature of Special:Watchlist broke on Commons and Meta at some point during the last few days.
Some pages changed after my last visit are not bold, and some that I have visited after the latest change are still in bold. All my watched and changed pages seem to be correctly bolded in Recent Changes however. Both the expanded and normal watchlist show this bug. The <strong> tag or its absence seems random.
I don't see any pattern to this, and apparently it's only happening to some users, not all. See also http://commons.wikimedia.org/wiki/Commons:Help_desk#Changes_don.27t_get_marked.