Here I posted a reply to some old thread. I see:
Not believing my eyes, I clicked my browser’s Refresh button, thankfully now seeing
I’m saying I should see that in the first place, without needing to click Refresh.
Yes, this is easy to miss while testing, because you need a thread from at least a month ago.
It’s easy to test on https://try.discourse.org/ 
I can repro it:
After posting, on the timeline, the last activity indicator and number of posts are updated, but the date of the last message isn’t.
At first, I thought it was because the timeline didn’t scroll to my post. In this regard, the shown date (Jun 14) was right.
After scrolling down to my post, it didn’t update and kept showing Jun 14 instead of show Aug 7, until the page is reloaded.