if you check the conversations where this happens (by going to the conversation, tap on the person at the top and go to "info", is the "hide alerts" option checked?
Because that's what I discovered did this to me. Yes, it disables the sound, the lockscreen thing, every aspect of getting notified of new texts.
It seems that they added this new feature, to hide alerts for specific people -- that's fine by me -- but the problem is that it randomly activates after the update, for certain conversations. I believe if you uncheck the option though it should fix it permanently for that conversation thread at least.