Old post indeed, yet I've hit the same thing recently. I'm using a Powershell script to install updates against a batch of servers, and I've seen that even if all is installed successfully, the "Updates were installed" never gets updated.
I went ahead and contacted Microsoft Premier Support and the technician there and - after checking with someone who is actually involved with developing the Windows Updates component - he told me that this is by design. What happens is that the date will
be updated only by Automatic Updates or during an interactive session, and updates processed through other means, such as scripts, don't get to trigger this. The reasoning behind this eludes me (a user can select to install one update only and leave the system
exposed vs. a script/SCCM patching 100% - the first would be "good" and second "bad" according to this logic).
I agree that taking a look inside Control Panel and seeing 2012 as the year when "Updates were installed" is a big red flag - especially for an auditor (not to mention the explanations one has to give them for reassurance), but there's always the
Powershell one liner which will yield when the last update was installed:
(Get-HotFix | Sort-Object InstalledOn -Descending)[0]