I haven't quite managed to figure out a pattern due to lack of testing/observation opportunity, but I have General as a bookmark, and it might be related to it being at the top, but often there are new threads in General, but it doesn't show it (in white text, as all other bookmarks do). One time it showed new threads but I didn't go there, but closed the browser tab with Spectrum, re-opened it and the marker was gone, but when accessing General, two new threads were orange-dotted there. There's no default selector on the top bookmark, so it doesn't look like that's why this issue occurs.