Urgh, it's still glitchy this morning, and completely random! Roll it back, roll it back, it's rubbish!
I'm the last poster on a couple of threads, in which case I get no preview and clicking takes me to the top of the thread. On some, if I'm not the last poster, it shows the last replies with previews as normal. Clicking on the poster as we previously did might or might not take me to that post, or to the top of the thread (adds an element of excitement, but not really workable). Then there are threads where the only poster showing is a randomly selected previous poster, not the last poster, and no indication of other thread activity.
Do you have a test environment, MNHQ? Because I think you might need to think about investing....