Unread Posts link - unreliable results

Unread Posts link should list all new posts for every replied topic at least.

However it does not. Sometimes show from one, but not from other topic. Sometimes when open that forum section and after click on "Unread Posts" link it link these missing posts as well.

It seems to me that feature is broken.

Default settings used.

However, whenever something is changed related to forum behavior and saved, after some time is reseted automatically.