We are talking about Assam not other states. If you are saying illegal migration has happened there must be reflection at census data if it has happened in big number. No such record is there.
Most importantly if any mass scale migration from Bangladesh to India has happened you would see in each and house there is someone in India which you will never find. Now 12-13 million Bangladeshis live abroad and most houses has someone who lives broad which is not the case in case of India.
Look at the article. So many stats with explanation is there.
This is district wise growth rate of all religious groups between 1971-1991.