If that's true of Devils and Fiends too, why doesn't that manifest in Tieflings?
(Note, this isn't directed specifically at you, Sozz, the quote is just the springboard because I keep seeing this and it bugs me...)
(Side Rant)
Okay. Let's be very clear about this, because apparently, despite it being mentioned *Many* MANY times, there are people out there who either refuse to listen, or wilfully ignore it.
Tieflings. Are. Not. Fiends.
Tieflings. Are. Not. Devilkin/Demonkin/Hellspawn.
Tieflings. Are. Not. Inherently. Evil.
For the love of the light, can people please get this through their skulls.
Aasimar are not Celestials; they aren't inherently good in their essence. Tieflings aren't fiendish; they aren't inherently evil in their essence. They are, quite literally, just the positive and negative equivalents of genasi -plane-touched, NOT plane-controlled or plane-dominated. Their plane-touched nature does not control or align their essence and core nature in any way whatsoever. It is a touching on their being, one which they may view in good or bad light, and one which other entities may try to take advantage of in various ways, but they are still mortal humanoids Native to the material plane. If you cast Banishment on an aasimar, it doesn't go to the outer planes - it goes into a pocket demiplane, because the material plane is its home. If you cast Banishment on a tiefling, it doesn't got to the hells or the abyss - it goes to a closed demiplane, because the material plane is its home.
There are MANY ways that one can become plane-touched, and by ancestral birth is only one of them. You do not have to have a lineage connected to it to be born plane-touched, if other events are the source of it; you don't even need to be born plane-touched to become one. People can become plane-touched by events in their lives, by being granted boons from greater beings, or even just form being in the wrong place at the wrong time.
Mayhaps your ship is caught in planar incursion where the realms brush, out at sea, and you barely survive; wracked by the wild and chaotic energies that sunk your ship, when you drag yourself out onto shore, you realise you only did so because you were breathing under the water for many long minute... and you find that you are now a water genasi - plane-touched by the plane of water.
This is, in fact, what happened to many people in Elturgard, during the descent; people who were not tieflings, who were born as members of other races, lived lives, grew up, and then were caught in the descent and nearly lost their lives; some of these people, when the city was returned to Toril, found that the planar crossover had changed them, and that they were now tieflings... They didn't become evil, or even evil inclined, they were just peasants, before the descent, and after it as well... now with a different look, unexpected abilities, and a whole lot of sudden prejudice to deal with, through no fault or action of their own (at least in some cases; other cases are the inverse, and they did make deals... you don't know, nor more than you ever know for anyone else; books, covers, judgement, etc.).