Svg Z-index. Which use the CSS Box Model. An element with greater stack order is always in front of an element with a lower stack order.
With SVG you have to rely on the order the SVG child elements appear. In SVG the element order determines z-order. So basically there is no z-index for SVG it uses the painters model.
And that element is covering up your checkYellow The z-index attribute will only be available in SVG.
That one is then rendered topmost visually. The stack level of the box in the current stacking context. This property is useful if you want to create. As in which one appears as if it is physically closer to you.