In reply to Andrew.

I could not reproduce the z-index issue. Tested with menu:

However, if I removed the z-index: 1 from the Heading, it seems to hide the background. Of course, there could be differences in our themes that is causing this. Most of the code is the stock wp-block-library stylesheet, but I do not use core's theme.css.

This is why I am sometimes reluctant to post CSS solutions. There are so many variables between designs that nothing is ever 100% guaranteed.