Why has this "details" accordion appeared in this view? I have a view that uses the table format and the "group by" setting.

I've just updated to the Drupal 8.9. Suddenly, each group has this empty Details accordion.


  


It wasn't there before.

Why did it appear? How can I get rid of it, other than removing it with CSS?
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.