Difficult one to replicate.

Writing mod which reads in rss feeds so it currently posts a feed item, with a past date.
And thus need to resync the forum(s) so it doesnt have todays date as when the topic/thread was posted

However on the forum home page, (and by extension a forum display page with a sub forum in it)
The last post details show the details of the Highest post id for that forum, rather than the most recent post time, when resycn occurs/is triggered, either from the acp or by calling the sync function