Eater of Worlds quest stuck / massive exploit possibility

I failed my first fight against Eater of Worlds, ran another map with his influence and got a second invitation and killed him.

He dropped a unique and a voidstone and I got the achievement, but the quest did not finish, I didn't get an atlas passive, didn't get the Eater of Worlds 14/28 mission counter on the atlas screen and didn't unlock the favourite map slot.

The game obviously keeps falsely assuming I failed again, and gave me another invitation after only one map with Eater of Worlds influence. Now I ran this third invitation for testing whether I did something wrong, but to the same result: I got a unique drop but the quest doesn't finish, no favourite map slot, no mission counter, no atlas passive.

Probably this could be abused to get Eater of Worlds invitations for every single map, in case somebody wanted to exploitatively farm his uniques.

However I did not get any currency from my two kills.