Use with Drupal: Memory game does not honour style hook

I am using H5P very successfully with Drupal 9 to create an educational site.

I routinely use the custom_h5p_overrides_h5p_styles_alter Drupal hook to modify the CSS style of components and I do this successfully with all sorts of components using...

function custom_h5p_overrides_h5p_styles_alter(&$styles, $libraries, $mode) {
  $styles[] = (object) [
    // Path relative to drupal root
    'path' => \Drupal::service('extension.list.module')->getPath('custom_h5p_overrides') . '/css/custom-h5p-overrides.css',
    // Cache buster
    'version' => '?ver=1',
  ];
}

However, no matter what I try in my overriding CSS file such as (for example to remove the card roundedness) - even with some !importants in place.

.h5p-memory-game .h5p-memory-card .h5p-back, .h5p-memory-game .h5p-memory-card .h5p-front {
  border: none !important;
  border-radius: 0 !important;
}

Yet, for other components such as multi-choice, override works a treat, for example (snapshot from the Chrome Inspector) ...

.h5p-multichoice .h5p-alternative-container {
  background: none;
  border: none;
  box-shadow: none;
  display: inline-flex;
}

Can anyone help here?

Thanks