From here, notice how the sidebars for these three sites are a tad different, notably, Math.SE. I'm not sure if it's just me, but Math.SE clearly shows whether or not a question has been accepted or not in the sidebar.
I decided to have a gander at the other sites, because I wasn't sure if it was just a bug that wasn't properly affecting Arqade or StackOverflow. These are the sites I noticed don't have the questions in the sidebar properly "marked":
Sites that do reflect if questions are answered or not (in sidebar):
I'll update this list as I find more, but can this be fixed?