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Slight spoilers ahead.

I think it is a bug, and is probably due to a problem I encountered throughout my playthrough:

One of the Source Hunter's personality was entirely controlled by me, and the other had the 'loyal' personality AI, which meant that they are 'supposed' to agree with whatever the controlled character chooses. However, during my playthrough I came across a few conversations where the AI still picked a different dialogue choice. I noticed that this happened when the dialogue choices didn't stay in the same order for both characters' dialogue box. Example:

Character one:
1. The day is nice.
2. The day is bad.

Character two:
1.The day is bad.
2. The day is nice.

The 'loyal' AI (character two), picks the same NUMBER in the dialogue tree as character one. Meaning that despite following the AI path, it is still the incorrect choice and character two disagrees with character one. This becomes more confusing if we bring in the traits that start to get all mixed up from these incorrect dialogue choices since the traits are also sometimes incorrectly assigned!

So, despite that, I tried to walk around the AI's dialogue bugs so my characters would agree on everything. But it appears I missed something or was supposed to let the AI disagree because I got this ending with Zixzax where he says that one character wished for the others affections but the other just left (how the game came to that conclusion through the conversations between the source hunters I don't know).

Since I roleplay my characters a lot, this was an extremely dissatisfying conclusion to my close to 100 hour playthrough of the game. :/ And even in the very last conversation between the Source Hunters outside 'The End''s door, the AI disagreed with me, cementing my worries that the AI's pathing would screw up how the characters' epilogue plays out.

This had been a problem in the original game with the dialogue tree and the AI and I was hoping the AI and the dialogue trees would have been fixed with the enhanced edition :(. I was lucky enough that I got the ending I wanted on my first playthrough of the game in the original, but I am extremely disappointed that I didn't get the ending I wanted again due to bugs in the AI. Is there even a guide one could follow to fix these issues when the dialogue AI is bugged like this?

I certainly hope that the AI dialogues are worked and properly tested from scratch for Divinity Original Sin 2 if there is a similar system :/. I love the game and things like these hurt the experience a lot.