I have missing notifications all of the time... pretty much always have. I don't know if it's the newness of the threads, exactly. That's more coincidental in my eyes. For me, I ALWAYS stop getting notifications if I either ignore a couple of them from a particular thread, or if it has simply been a little while since I last viewed it. That's the deciding factor.
An example... the "what are you playing" thread has always been hit or miss on notifications for me. In a matter of maybe 5 days, another page may pile up without me receiving a single notification. But then, as soon as I open the thread again and somebody posts, I see the push. And this will go on for a few days before going dark again.
Worth mentioning, it only happens with replies to the thread itself for me. If somebody quotes, tags, or reacts, it seems like I always get that notification.
This has me thinking maybe it was set up to work that way by default, to avoid people essentially being bombarded when watching many active threads at the same time. It only shows you general replies for threads that it gauges you are actively engaging in based on the data you feed it. Probably a setting that could be changed, but then I don't know it should be in that case. The only thing suspect for me is that the cutoff was supposed to be 6 months, but for me it doesn't even take a week to stop seeing the notifications.