The difference in the original Note charger is that it supports fast charging. Other 2a chargers will work--they just won't support the fast charge feature.
You may have been using something with a different rating?....but I have used non-adapative chargers on the Note 4 that are rated at 2.0 without any problem. Of course, I was using the Note 10.1 2014 charger for that purpose. It doesn't charge as fast AS adaptive charging, but it worked.