This has never happend to me but don't be alarmed because probably your tamagotchi was at his maximum training and when you disiplined he knew he deserved it and never lost a happy heart that's possibility #1
#2 is that it was a glitch
#3 Is probaly you took excelent care of your tamagotchi and it never affected his happiness
Depends on all of its training i guess?? Its like when your older and your mother shouts at you , you know you deserved it because you did something wrong..