It is tricky to debug this without knowing the topic that was causing the issue. If you notice the issue again, can you try to make a note of which topics it’s happening on?
Based on your settings on Discourse, you should see a blue dot after the title on the Latest list for topics that were created in the past couple of days. These topics should also be listed in your New topics list. If you see a blue icon with a number in it in the Latest topics list, this is supposed to indicate that there are new posts in a topic that you are watching or tracking. These topics should also appear in your Unread list. You will be automatically set to watch a topic if you’ve spent 4 minutes reading the topic, so possibly this is what happened.