Details element not correctly handling the #open attribute.

could you please assign tree value as TRUE OR FALSE and retest again.

   $form['more_details'] = [
      '#type' => 'details',
      '#title' => t('More details in here'),
      '#open' => FALSE,
      '#tree' => FALSE,
    ];

Sorry, I'm going to close the "bug". When I did my initial test, I was doing it on a block inside layout builder, with a lot of custom modules enabled. Trying to replicate the bug on a clean instance, I was not able to replicate the issue.

I should have done the test on a vanilla Drupal before opening the bug, sorry about that... lesson learned.