Condition to enter the Black Core bugged?

It seems the condition to be able to enter the Black Core is not the turn in of the organs to Piety, but that the three mini-bosses in the Harvest are dead. This seems wrong to me, and the quest text says otherwise as well.

Took my Duelist to cruel Malachai yesterday, made my way through the belly, past Piety, killed the three guardians and turned in the organs. Then decided to get another level before Malachai and made a quick trip to the Dried Lake (which was also worth it because it dropped me a 5-link ^^). Afterwards, the Harvest was of course reset, but Piety was gone.

On clicking the entrance to the Black Core, I got the message that I'm not allowed to enter as long as the guardians live. Quest Text (U): "Piety has revealed that Malachai is in control of the Beast, and is residing in the Black Core. Find your way inside and kill him."

Porting back to Highgate, the quest text changed to "Piety has used Malachai's organs to gain access to the Black Core and offered to help fight him. Go inside and kill Malachai."

The quest "Corpus Malachus" was marked finished at this stage.

I could enter the Core after I killed the guardians a second time, but IMHO this shouldn't be necessary, no matter if the area resets between the guardians-kill and the Malachai-fight. If "Corpus Malachus" is done, the entrance should be open.

Okay, with that reasoning it makes sense (though, who in their right mind would farm Malachai of all bosses?). It's not logical, though, at least not in regards to the quest progression, especially as the quest stage changes randomly between the Harvest and Highgate as long as you haven't finished the act.

If the condition to enter the Core is the trio kill and not the turn in of the organs (and in regards to the non-farmability of Malachai that's reasonable, I admit), the quest should reflect that in some manner.

Or make it so that the first time per difficulty you try to enter the Core, you can do it after you turned in the organs, no matter if the zone reset or not. After that, when it comes to farming, there are no quests any more anyway to bother with.