I just checked and I only added the category description to the H1 code in the Header.php file.
Here’s the original H1 code from the Spacious theme:
<div class="post-title-wrapper">
	<?php
	if( '' != spacious_header_title() ) {
	?>
	<h1 class="header-post-title-class"><?php echo spacious_header_title(); ?></h1>
	<?php
	}
	?>
</div>
And here is the same area in my child theme:
<div class="post-title-wrapper">
	<?php 
	if( '' != spacious_header_title() ) {
	?>
	<h1 class="header-post-title-class"><?php echo spacious_header_title(); ?></h1>
	<?php echo category_description( $category_id ); ?>
	<?php
	}
	?>
</div>
I wouldn’t think adding a line for the description would change the code enough to cause a glitch in the H1 titling of the plugin. Thus, I am assuming it is either something else or the theme is breaking the plugin.