#default_value Not Working In Select Field
On Drupal 8 form, I was having an issue with Firefox that was caching the value of a select field, selected by the user before refreshing the page.

It was basically overriding the #default_value of the field.

My solution was to Turn Off Form Autocompletion (https://developer.mozilla.org/en-US/docs/Web/Security/Securing_your_site/Turning_off_form_autocompletion). 

You can do it at the level on the input if that's an input, but since it was a select in my case I had to do it at the form level :

$form['#attributes']['autocomplete'] = 'off';