Read More buttons not showing

I posted this same issue a couple of days ago but now cannot find the thread, either in the main forum or in the tickets opened by me under my profile. Weird.

Anyway, to repeat:

I'm using a custom excerpt on blog posts. The read more buttons on the homepage/category pages disappears when this field in Gutenberg editor is filled with a custom excerpt. Please advise how to get the buttons back?

I'm using the latest versions of WordPress and GeneratePress.

Settings -> Reading -> Summary is enabled.

I've already tried using the CSS provided in other threads/help articles, e.g.

add_filter( 'wp_trim_excerpt', 'tu_excerpt_metabox_more' );
function tu_excerpt_metabox_more( $excerpt ) {
$output = $excerpt;

if ( has_excerpt() ) {
$output = sprintf( '%1$s <p class="read-more-button-container">%3$s</p>',
$excerpt,
get_permalink(),
__( 'Read more', 'generatepress' )
);
}

return $output;
}

But receive a warning that this CSS is not valid and I need to correct it before publishing in case it breaks the website.

Thanks, looking forward to hearing back from you.