Chances are that you also added the
position: relative beside the
z-index value when you wanted to change the element's position along the z-axis.
position, you are creating a stacking context, and without a stacking context, the
z-index does not take effect. But that's just one of the many ways to create such contexts.
It is also created whenever you set the
grid, or use the less known
The next time you automatically add the
position, pause for a moment because it might not be necessary to do it and can save you from potential undesired side effects. Or better, set up a linter to catch it.
If you want to a deep dive into
z-index, then check the What the heck, z-index?? article.