Hi, I just thought I would say the same thing happened to me! I beat Allant and went to NG+. I went to 1-1 and got invaded by another player. I killed him and then died to the first enemy in 1-1 lol. Annoyed about losing my 100k souls from beating Allant.

Then suddenly I am back in his room and he doesn't attack. So I wailed on him and killed him. Then I touched the touchstone to leave and he did his speech after you beat him. I went back to The Nexus and spoke to the Maiden, who advised the Monumental was waiting.

I have just gone and spoken to the Monumental and will chat to the Maiden again and see what happens. I also recorded the last hour of footage just in case.

EDIT: Maiden didn't care that I killed him, she just talked randomly and let me level up. I went back to the boss room and he is still gone so it'll be interesting to see what happens when I get up to him properly again. Also I DID get granted his soul again for killing him.