Modal unresponsive after backgrounding app

If I open a modal and then background the app (tested on iOS) and then bring the app back to the foreground - the modal is still displayed but is unresponsive to any touch events. I have to restart the app.

How do I prevent the app from freezing when returning from the background?

Modal presentation:

<ion-modal-view ng-controller="txDetailsController" on-swipe-right="cancel()">
  <nav class="tab-bar" ng-style="{'background':skin.view.navBarBackground}">
    <section class="left-small">
      <a ng-click="cancel()" ng-style="{'color':skin.view.navBarButtonTextColor}">
        <i class="icon-arrow-left3 icon-back"></i>
        <span class="text-back" translate>Back</span>
      </a>
    </section>
    <section class="middle tab-bar-section">
      <h1 class="title ellipsis" ng-style="{'color':skin.view.navBarTitleColor}" translate>
        Transaction
      </h1>
    </section>
  </nav>

  <ion-content class="modal-content">
...
<ion-content>
<ion-modal-view>

To open the modal:

  this.openTxModal = function(btx) {
    var self = this;

    $scope.btx = btx;
    $scope.self = self;

    $ionicModal.fromTemplateUrl('views/modals/tx-details.html', {
      scope: $scope,
      backdropClickToClose: false,
      hardwareBackButtonClose: false,
      animation: 'animated slideInRight',
      hideDelay: 500
    }).then(function(modal) {
      $scope.txDetailsModal = modal;
      $scope.txDetailsModal.show();
      $rootScope.modalOpened = true;
    });
  };