Re edited for clarity.
Its definitely lost the plot!!
Old posts are suddenly showing as "New"
In some areas such a Regional Chit Chat and Brand Banter we have lost the option to "Mark Topic Read"
Where it is available on some topic areas using mark "Mark Topic Read" does not clear all the new post flags that have been created.
Why did it need to change?
My son has been a web site developer for several years, and he tells me the best practice is to run any updates on a dummy site or environment to prove all the changes work on all platforms before it's released to the main websites, or its posted a Beta site for a fixed time and members are invited to try it on the understanding that it may still contain errors, which will be addressed when they come to light.
This is not the first time that changes to PC forum have been implemented and, the user experience is severely hampered, even compromised on one occasion where member details were displayed than should have been. I know that PC forum is not the highest priority in Haymarket's empire, but all too often changes are implemented that clearly have not been thoroughly tested.