The 25% damage buff does not linger for 30 seconds after a unit leaves the radius of the Kurultai.
@SavageEmpire566 , you said that this is just an issue with the visual of the glow in the attached post and then locked the thread, but in the current live patch this is not true.
Here is a video showing the proof of this. In the video, the archer hits a longbowman with the Kurultai buff for 7+4=11 damage. The archer then leaves the Kurultai radius and then shoots a crossbow for 7+2=9 damage.
Kurultai IS BUGGED and so 100x worse than before - #8 by SavageEmpire566