I had thought that this long-standing bug was finally fixed in 7.5 but it doesn't seem to be the case here.
If I view a previously visited thread in Threaded Mode where I had read, for example, all the replies of the original post and where there are now several new replies, the unread (yellow) folder icon is displayed correctly for those new replies. HOWEVER, if I view the first new reply all the other new replies then display the "read" (white) folder.