latest WP update they changed the way cover block are displayed – which includes poor CSS that sets a 100% + padding but they neglected to add correct box-sizing. Its a known issue that WP should fix.
In the meantime add this CSS to fix the issue:
.wp-block-cover {
box-sizing: border-box;
}
Or you may want to check out GenerateBlocks which allows you to create much more advanced layouts: