Managed to work around the issue by importing my child theme’s stylesheet and creating an editor stylesheet to add anything that was missing.
Added the following to my functions.php, for anyone else having this problem:
// Gutenberg custom stylesheet
add_theme_support('editor-styles');
add_editor_style( 'style.css' );
add_editor_style( 'editor-style.css' );
add_filter( 'pre_get_posts', 'exclude_category_home' );
This may be the wrong way to do it, so will wait for feedback form support before marking this resolved.