Tieflings suffer from an incredibly mixed origin, in terms of writing, not lore. The writing has changed wildly over the years, and been retconned (looking at you Mordenkainen's Tome of Foes).

In 3e, widely considered to be the golden age of planetouched, they were either locked to evil, or locked to chaotic, depending on your specific source. But, at the same time, Tieflings were also not that far removed from their fiendish ancestor, at this time it was possible to breed out the fiendish blood, so you could have people looking entirely human with this distant ancestry, and Tieflings suddenly born to normal human parents because they had a fiend way back in their family tree. And, just for clarification, the most fiendish blood a Tiefling could have was around a quarter, too much more and they would be born as a Cambion instead.

Contrast this with Tieflings today. Due to God shenanigans, it's no longer possible to breed out the fiendish blood, the offspring of a Tiefling is always a Tiefling, but at the same time no Tiefling we meet has a true fiendish ancestor within ten generations of them. This goes extra for the Tieflings in this game, because they are refugees of Elturgard, which fell into Avernus, and some of the people who were living in the city were transformed by the ambient energy of the place. They actually have NO fiendish ancestor, they are what we call spontaneous planetouched. Given the way things like this work, I imagine there was also a number of Tieflings born to non-planetouched parents in Elturgard, also spontaneous planetouched.

Now the special thing about planar energy is that it only really affects people when they are on that plane, or are near a planar incursion involving that plane, so some of the transformed people might have had an alignment change, but it's just as likely that that alignment change was reverted when they came back to the material, even if their appearance wasn't. There are many planes that work like this, they change you while you are on that plane, but leaving that plane will revert some of the changes, it's why traveling the planes is serious business, there's the possibility that you won't be the same person on your return trip.

But, what I'm getting at, is that despite the more dramatic and demonic appearances of Tieflings going from 4e onwards, most of them are not at all closely related to their fiendish ancestor, if they even have one at all. They have all the looks, but none of the actual fire and brimstone. They are like a selzter water version of a fiend, vaguely fiend flavoured, but not actually fiendish at all.