I think it has been happening for more than 12 months. On one of my sites I had a perfectly innocent links page called links.html. The links were in context, relevant and carefully selected as they could have been of use to my website visitors.
I renamed the page added some more content and after a few weeks (I can't remember exactly how long) the PR was restored. This did not prove anything conclusively but it suggested to me that Google will disregard pages that it thinks just contain links.
... it suggested to me that Google will disregard pages that it thinks just contain links.
This could be true, or maybe they're not disregarded within the system, but rather just have the TBPR display disabled? Did you notice any change to the traffic or rankings of the page at the time the PR disappeared from visitor view, or did those stats stay the same?
Maybe we're saying the same thing with different words and what I'm saying is what you meant by disregards?