Users report an issue where the sidebar shows incorrect new topic counts per category, such as “1 new” or “New (7)” despite no visible new content. RGJ first noticed this with the “bug” category, suspecting it might be related to closed topics, but the issue resolved after creating a new topic. sam confirms the bug is hard to reproduce. tvavrda explains a similar case involving a “About [Category]” topic in a subcategory, where clicking the topic clears the notification. HamMan2118 also experiences this, showing discrepancies in both “New” and “Unread” counts, with images and a forum link provided for reference.
I have seen this when I created a new sub-category. There is always this “About [Category]” topic, which is kind of semi-visible.
I have the main category in the sidebar. After creating the subcategory, the sidebar showed “1 new”. I clicked on it, showed the main category list and – nothing in it. As I realised what I have just done, I went into the subcategory, clicked on the “about” topic and the notification went off.
I don’t think this is your case but maybe it will help.