I believe you're experiencing the recent change to database-based tracking of read posts.
Database (no automatic forum marking) - this option uses the database to store thread and forum read times. This allows accurate read markers to be kept indefinitely. However, in order for a forum to be marked read when all threads are read, the user must view the list of threads for that forum.
Those who like the automated neatness of things being marked read en masse without reading them will not like this change. Those who want to read one thread now, come back later, and not miss a thread that was automatically marked read will prefer it.