I see you are using "Button" as it's type. But, based on Blizzards code it is a "Frame".
Lua Code:
<Frame name="MaximizeMinimizeButtonFrameTemplate" parentKey="MaxMinButtonFrame" mixin="MaximizeMinimizeButtonFrameMixin" frameLevel="510" virtual="true">
Link to line in file :
https://github.com/tomrus88/Blizzard...15C2-L1615C155