I have the same experience.
One and half month ago, I was told that my case was assigned to an officer in Jan after they received my response of RFE. However when I called again last week, l was told that my case has not been with any officer yet. She also mentioned they are working on the backlog.
My best hope is that the previously assigned officer is not working in approving 485 cases now. So INS return my case back to pipeline, and waiting for other officer to pick up. That also explains why my case went with an officer for so long time and still not approved.
My INFO:
RD: Sep, ND Oct, RFE Dec waiting....