Tormented bosses grant one, lilith grants one and season journey grants one.
Got my first one from season journey, then nothing from tormented boss. After the lilith patch fix got one which must have been for tormented bosses as i never killed lilith before, then did lilith and got my third one. And afaik its not even t4 bosses as i did not do one at this stage and as well did not salvage anything by accident, this can be dismissed.