Issue exists in Mobiscroll v2.8.3 and v.2.9. If a picker is initialized with a third button:

.mobiscroll({
  preset: 'datetime',
  (...)
  buttons: ['set', 'clear', 'cancel'],
  onClose: function (value, btn) {
    console.log('onClose', btn);
  }
});

then after using a 'Clear" button the onClose handler will fire and it will output 'onClose undefined'. For 'Set' and 'Cancel' buttons the btn value is set properly.