Why has this "details" accordion appeared in this view?
I ran into this issue with a View as well. Fortunately, this issue was reported to Drupal Core, fixed and released in Drupal 8.9.2 and Drupal 9.0.2 all in under a month which is quite commendable.
I just tested this issue by upgrading a test to Drupal 8.9.2 which resolved this issue.