The best working theory is that if you complete the quest on a higher difficulty, the recipe doesn’t drop and you are never able to obtain the original lower level recipe (which is only a problem because you need to make that one to make the higher level ones).
I don’t think anyone knows for sure what causes it, but the only way to get around it is to get it on a new character and re-doing the entire quest.
You can also get normal invokers as a drop by joining friends in their quest dungeons as an alternative to make foul/tormented which is what I did, but you still don’t get the original recipe and can’t farm the wt4 version for 800 ilvl gear on your own if you wanted to do so.
What the original poster of this tragically misinformed thread is saying is completely false. They are entirely wrong but detached from reality and unable to accept that they are wrong.