最近、未読メッセージの青い数字が（本来表示されないはずの時に）表示されるのはなぜですか？

[Jonathan5]

@Mevo

it represents “unread” messages

Here is a post I bookmarked yesterday. The blue circle is “new” and the grey one is “unread”:

@codinghorror

Grey means unread but not new. Posts you could have read the last time you were in the topic, but chose not to, for whatever reason.

Blue means new posts. There’s no way you could have read these posts, since they did not exist the last time you were in the topic.