So,if russia & Finland signed a perpetual non-aggression treaty, and that is good enough; why should it trouble Putin if Finland does join NATO?
A promise to not invade Russia, and because Finland is not about to invade Russia. joining NATO need not be mutually exclusive.
Only way Putin would have to worry, is if he plans on attacking a NATO country. ...And I suspect that with his ongoing situation in Ukraine, he will not be greatly adventurous at anytime soon.