When you use the 'Delete All Cookies' option it also removed the cookie for your last visit date used for read/unread posts. This means that the forum no longer has a last visit date and will therefore show posts from 'Yesterday' by default on the Latest Posts Page.
We do see time and time again that when forums are updated to a new major release that members will say things like, 'it's running slower', 'unread post notifications', 'auto logins not working', 'email notification not working', or anything else. The problem is that none of these things have changed between version 9 and version 10 but the people who complain tend to make allot or noise about this while most other stay quite, which tends to give a biased opinion that everyone is having issues, when in reality these things remain the same as they always have.
The unread/read post notification system has not been changed for version 10, and we have not heard of anyone else with an issue who hosts their forums with us and we do host many 1000's of Web Wiz Forums websites. I have also kept checking in with your forum myself and have not seen any issue with the read/unread post notification system.
I would suggest just leaving things and see if any members contact you in a few weeks, as for now as the code is not changed and your forum is optimised there is nothing we can do.