So, this is something that bothers me. There is a quest in BG 3 to rescue/save all the Tieflings across ACT 1 and 2. You can do that with any character origin, except one: "The Dark Urge."

I won't spoil exactly what happens, but why do we lose a unique very rare piece of armour due to a events that happen? Why isn't there someone else that can give the reward prior to the end of the quest? It's even more glaring, as there is a safeguard for an event like that in Grymforge, due to all the lava there.

There isn't one here. In fact, the only way to get the item is to use extremely metagamey option prior and some shenanigans to prevent the event from firing in the first place.

Just to be clear, I am not against the event or the origin, but annoyed by the fact that we lose out on stuff, due to one single NPC. I wish there was someone else to reward the player instead.

It doesn't help that playing evil in BG 3 is simply worse. You lose out on content, magic gear, quests, etc. Evil side has got so much less stuff to do, it's not even funny. So, my suggestion is to insert a backup character to give the player the reward, once you finish the quest.