Should it be allowed then? It already breaks a function, and if more users start using this kind of thing to improve visibility of their threads compared to others the function will be even less useful.

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.
Thanks. I hope this thread is enough but I also will send him a message.