According to Theymos he has been making some little changes that might lead to several malfunction or bug.
This could be one of those bugs which he said we should report if found. Since the behaviour is very unusual, unlike every other thread it's supposed to respond accordingly by marking read but doesn't which means it's a bug.
Here is Theymos statement regarding this.
I've been making a number of little tweaks over the past few days which could cause hard-to-predict bugs, so let me know if you see any other buggy behavior.