After upgrade to cms 9, MainBody is empty in edit mode

Hi,

I am updating a site that was written in 7.X, it is currently running in 8.11, and I did the update to bring it to 9.12. Everything at this point seems to be running correctly. However, when one enters the CMS to edit information, then the body of the content is empty.

I steped through the request, and I found that in the initial process the PageData object has MainBody content. Yet at a later time, this appears empty.

What my app does is that it collects the information, and at some point in the processing of the page, it calls one of these.

 base.OnActionExecuting(filterContext);

 base.OnResultExecuting(filterContext);

After these filters gets called, when the breakpoint comes back again, the MainBody property is now null. 

Any ideas of what could be going wrong?