Yeah, Rick would be the one to comment. I've been testing it and haven't had any real issues myself - but the new post tracking feature was redone in 6.5.
From the changelog:
Totally reworked the new post tracking feature. This new tracking method is extremely efficient and much faster. The U_TempRead field is no longer used and has been removed from the database. This new method will also allow us to do caching of some of the pages in a future version.
Along with the new post tracking feature, the new post designator was changed to a colored folder on the postlist screen. The colored row option is still available by the original .css classes but by default, these classes will now be set to the same as normal colored rows.[/]
So it's very possible there are bugs yet to be uncovered.
You *just* posted it over there - Rick has been pretty religious about tracking down the bugs and responding to everything over there.
The only other thing to check would be cookie related issue - does this happen to just you, in just one browser? Have you gone through the process of trashing your cookies and other normal new post troubleshooting steps?
This post outlines Browser Issues and how to solve them.http://www.ubbcentral.com/support/faqs.php?type=browser
But if it's widespread, then I'd say a potential bug.