RSS feed issue

Some of members of our forum reported weird issues with forum RSS feeds.
They reported that in most cases, feeds aren't updated properly and that some posts are missing.

At first i couldn't replicate the issue (worked fine in firefox using sage++) but then i checked opera and IE and they both showed what our members are experiencing.

Since the old feex file (version 9) worked fine i was able to find the culprit for such behavior; the <guid isPermaLink> tag is what was causing this strange behavior.  After replacing the new (v10) code with the old one (no seo and rewriting) the feeds seem to work fine.

Any idea of what is causing this ?
(it isn't an critical issue)