I have and it’s still not completed on the alt either. There’s some game event or mechanic that must be completed as well. I am saying the latter as I’ve done all of the DLC events except those specifically tied to playing a Spiritborn which I will not play this season.
Edit: Correction, doing the alt works. Started another alt but this time I went down to Nehantu and started the quest right away, without levelling or doing any other quests. This time the Coiling Faith achievement was awarded. So there must be some other event, quest or achievement that if done prior to this side quest prevents it from being recognized as completing. Players beware, it is still bugged but can be resolved.