Karl,
I found a bug report on Facebook that describes your problem. There are many, many others experiencing this as well. It seems as though some sites it just started happening with for new content. Switching your URLs to be SEF changes all of your previous URLs, which made this bug kick in for all of your content:
developers.facebook.com/bugs/496305530382482
The good news is that the issue is widely reported and has a priority of "High", so it should hopefully get fixed pretty quickly. I'd still recommend hitting the "Does this repro describe your issue?" link on the page above and putting in your site details. The more reports, the higher the priority.
Hope that helps explain and hope Facebook fixes this issue up soon!
Thanks,
Alex