Due to migration to a new caching strategy, a bug was introduced that would not change unread post count badge when the user reads an unread post.
Please note, that the product has an option Unread Post Count Cache (Minutes) in the option group Threads, discussions and conversations which defines how long unread post counts should be cached. If caching is not desired, and you want your users to always immediately see the number of unread posts, just set this option to 0. Caching is highly recommended for large websites, for at least as short as 1 minute, as executing an aggregation query on each page load may increase server load considerably.