Uninstalling module removes the commerce_coupon_fixed coupon type.

Uninstalling this module removes the wrong coupon type. See the hook_uninstall implementation.

/**
 * Implements hook_uninstall().
 */
function commerce_coupon_pct_uninstall() {
  // Make sure commerce.module is loaded so that commerce_delete_instances()
  // is available.
  module_load_include('module', 'commerce');
  // The function commerce_coupon_type_delete() can't be used here because
  // commerce_coupon module might not be enabled anymore at this point.
  db_delete('commerce_coupon_type')
    ->condition('type', 'commerce_coupon_fixed')
    ->execute();

  // Delete all fields that belong to the module's coupon type.
  commerce_delete_instances('commerce_coupon', 'commerce_coupon_pct');
}

Patch fixes the issue.