Hi there,
Using a checkbox works inside of a custom HTML block inside a secure block, but if you use the checked attribute inside the checkbox, it throws an error and blows up the page, not showing the entire secure block and any content inside it.
Warning: DOMDocument::loadXML(): Specification mandate value for attribute checked in Entity, line: 4 in /app/public/wp-content/plugins/secure-blocks-for-gutenberg/blocks/secure-block/php/class-secure-block.php on line 94
See gutenberg secure block usage with custom HTML below:

See error below:
