WP updated the Cover Blocks HTML and CSS to include 100% width + padding, but negated to add the box-sizing: border-box; It has been reported to their GitHub for them to fix.
In the meantime add this CSS:
.wp-block-cover {
box-sizing: border-box;
}
The upcoming GP 3.0 will automatically apply this to content globally so if developers get lazy the Theme will correct it.