SOLUTION:
Change this line:
"apiVersion": 3,
to this
"apiVersion": 2,
I think you are right about it being a gutenberg bug. The bar covers the scrollbar down arrow too in the editing window. I found a bit of a workaround by adding a special admin area CSS.
In my theme directory a file called style-admin.css
which contains the following:
#editor .postbox:last-child > .inside.acf-fields {
padding-bottom: 2rem !important;
}
Then add the following to my functions.php
function mytheme_admin_style() {
wp_enqueue_style('admin-styles', get_template_directory_uri().'/style-admin.css');
}
add_action('admin_enqueue_scripts', 'mytheme_admin_style');