Bad example.  People of Gomel are very Belarusian. People of neighbouring districts of Briansk region of Russia next to Gomel ran petitions on number of occasions to join Belarus.

But we had a separatist movement in western Polessia in the 90s. Poleshuks of Belarus, Ukraine and eastern Poland wanted to create a separate republic. The government and people of Belarus said Poleshuks of Belarus are Belarusians. That was the end of it.