CSS transform with z-index stutters!!

Here is the pen: https://codepen.io/Supersudo/pen/PoPweJL (needed to rerun the code)

I have a dropdown inside a dropdown, the when the second dropdown is minimizing, it will ignore the z-index for a second then go back to its style when the animation is done.

I noticed that this only occurs in firefox, I tried it in chromium and worked as expected.