8 - Register form automatically fills in input fields
I found a solution thanks to the Drupal Community

Apparently this is defined in the core of Drupal: core/modules/user/src/RegisterForm.php

The following should be set to FALSE

// For non-admin users, populate the form fields using data from the browser.
    if (!$admin) {
      $form['#attributes']['data-user-info-from-browser'] = TRUE;
    }


However it would be better to not mess with the core values and write a form_hook_alter instead in your theme file:

function MYTHEME_form_alter( &amp;$form, &amp;$form_state, $form_id ) {
  if ( TRUE === in_array( $form_id, array( 'user_register_form') ) ) {
    $form['#attributes']['data-user-info-from-browser'] = FALSE;
  }
}


This solved the problem.