Hi, I'm still investigating this but when I have errors in my ModelState, they appear to be lost after rendering a content area.
Walking though a debugger after a form submission, the modelstate has an invalid property "Category" with an error from a required attribute of "The Category field is required." (watch on ModelState["Category"].Errors.ErrorMessage)
I stepped past @Html.PropertyFor(m => m.CurrentPage.ContentArea) before my form with a single content block containing only one XhtmlString property. After that the ModelState still has the "Category" property marked invalid, but the Errors collection is now empty.