The icon and the thread title generally point to the same thing in the default style. The only case where there's potential to vary is if the thread was last read long ago (read marking data is out of date) or never listed as read. Those would both lead to the first post of the thread and the same thing should happen with the icon even with a slightly different URL.
I could only guess that there's an add-on/style interfering or data isn't being saved in the database properly (though I don't know why). Marking all forums read would certainly remove the issue, but I don't know if it would come back in a month or so.