Hi Ashish,
In your process RR is carried out after invoice is created or before that?
I do understand that you did not do RR for the postings happened in the month of dec15 and as in your post you had mentioned.
I think your customers billing date must on on 20th of each month.
What must happened is that your customer next billing date was generated for due list of JAN16 so when you carry out your RR in the month end this is reflected for Feb16.
Either you must delete the next billing date for JAN16 and with DEC15 (due for billing date) you should try the RR process and maybe your problem could be solved.
Thanks
JNM