Tony, I encountered this as well. Instead of the category filter use the "keys" to narrow the grid by category. I wrote this blog post a while back where I encounter the very same issue when using the viewItemFileService. http://notesspeak.blogspot.com/2013/07/creating-updatable-rest-service-for-use.html
I would consider using the json service instead with your REST, as I found it more reliable.
This question might also help you: XPages Dojo Data Grid and REST -- can you point to another application's view?