surrender treaty vanished

[Courtenay]

A surrender treaty with Prussia contained an enforced peace starting in Jun '08. In May 08, that treaty was still there on the view treaty screen. In June '08, I was very surprised to have Prussia declare war on me; when I checked the view treaty screen, the treaty had vanished. Repeating from the save game file
produced the same result. I have had many enforced peaces, and none of them behaved this way. What happened? Is this a feature I am unaware of, or is it simply a bug?

Thank you.