This happened to me after I tried to delete a post and got that thread_sort error (or whatever it is) that made us disable the trash can mod to stop the error. I deleted another post after that, and it updated the last post. Did you do a recount in ACP? That usually sorts that out!
I'll do that, but I thought it might be kind of a bug.
Maybe...I just figured it was a side effect of the conflict with the trash can mod!
All bugs relating to the new system are probably problems caused by other plugins screwing up. Just rebuild your counters and wait for an updated version.