themes-wordpress/blockbase/sass/blocks/_post-content.scss

// Needed until https://github.com/WordPress/gutenberg/issues/35267 is resolved.
.wp-block-post-content { // Only apply these margins in the post content.
	p.wp-block.wp-block-paragraph, // This selector has been made extra specific to override the block gap being set in the editor.
	*[class^="wp-container"] > * + p,
	*[class^="wp-container"] > p + *,
	p {
		margin-top: 1em;
	}
}