Z-index only works within its stacking context

A child element with z-index: 999999 will still render behind a sibling of its parent that has z-index: 2. This is because z-index creates a stacking context. New stacking contexts are formed by position + z-index, opacity < 1, transform, filter, and other CSS properties. The fix is to inspect upward through the DOM to find which ancestor creates the restrictive context. MDN Web Docs has the complete list of stacking context triggers.