I had exactly the same issue;got the phone and set it up,then out of nothing the led stopped working.
I did a full factory reset, and nothing, I tried testing it through the the Samsung service menu and nothing. Downloaded light flow app to test and nothing.
I'm very sad since I rely a lot in the led notification light.
I'm reluctant to believe the led hardware failed, seems to simple, I also believe it to be a software issue. Now that you say that you have had the problem twice I would bet on it.
Anybody else had the same?