Target yourself and type /disband or type /disband name. Passing of leadership before you click the disband button is also a good idea.
The UI disband button should have the exact same functionality as that command so im not sure how it could be causing you to target and disband a random member. Might have to fix the group window which i know very little about, my custom UIs never work lol, or try another UI to resolve that.
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