There’s actually two reasons:
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Its parent is set to show up behind the logo. Any z-index applied to elements within that parent element will only apply to other elements within that parent. All the elements within the parent will be stacked accordingly, then that entire element will be put behind the logo as specified by its stack order.
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A z-index only applies to elements with position of absolute, fixed, or relative. It does not apply to elements with static position.