Windows 7 Home Premium, Service pack 1. PM V6 build 3558.
I'm pretty sure this bug was introduced by build 3558 and the previous build I was using was alright.
Anyway, if I update using the Metadata (IPTC) template (crtl-i), I can see the changes if I view the metadata via the Metadata (IPTC) Info pane, but they don't show on the labels in the contact sheet. F5 doesn't help. The only way I can see the new data is by closing the contact sheet and reopening it.
The contact sheet updates if I enter the data via Info pane.