I have heard that if you roll a nat 20 on the last interaction with the brain, it at least has some lower health in the final fight.
I’ll need to check that. If this were visible in the scene, it would reduce the feeling of arbitrariness. BG3 otherwise does a great job with respecting player choices, so this feels really out of place.
To minimize the cost of changes, an entry in the battle log could give that hint.
Something like “The Absolute rips free of the bonds (200 damage)”