Finally, it happened. After migrating all markdown input filters to CKEditor, I can upgrade this site to Drupal 9 series. Gutenberg seems to be broken on Drupal 9.2 which affects me a little as I use it for authoring page content only. It is the best decision to choose CKEditor, which is on Drupal core, instead of Gutenberg for blog content.
Upgrade to Drupal 9.2