It's not that simple; in that part of India (the northeast), there is all sorts of dynamics between different ethnic AND religious groups. The Assamese versus the Bengali vs many tribal groups who speak various Tibeto-Burmese languages. Then there is various religions. Hindus, Muslims, tribal animists, various Christian groups, etc. Between India and most of Bangladesh there isn't much of a natural border. There has been en masse migration after 1947, 1971, and more recently.

The BBC had a good story about this recently: https://www.bbc.com/news/world-asia-india-50757093