Hi Sean,
I tried that exact code in 1.2 (with added step to create GDB) and the editOperation returns true (I assume success), but the new field will not populate. It just remains <null>.
I want to edit datastore data, not map data, so went for the callback approach.
See code below:
protected override async void OnClick()
        {
            //create geodatabase
            var cts = new System.Threading.CancellationTokenSource();
            var mva = Geoprocessing.MakeValueArray(@"D:\Temp", "Test.gdb");
            var result = await Geoprocessing.ExecuteToolAsync("CreateFileGDB_management", mva, null, cts.Token, null, GPExecuteToolFlags.RefreshProjectItems);
            //create feature class from template
            mva = Geoprocessing.MakeValueArray(@"D:\Temp\Test.gdb", "Test", "POLYGON", templateTablePath);
            result = await Geoprocessing.ExecuteToolAsync("CreateFeatureclass_management", mva, null, cts.Token, null, GPExecuteToolFlags.RefreshProjectItems);
            //copy selected features from a layer in the map to feature class
            mva = Geoprocessing.MakeValueArray(templateTablePath, @"D:\Temp\Test.gdb\Test");
            result = await Geoprocessing.ExecuteToolAsync("CopyFeatures_management", mva, null, cts.Token, null, GPExecuteToolFlags.None);
            //add field to test feature class
            mva = Geoprocessing.MakeValueArray(@"D:\Temp\Test.gdb\Test", "someID", "LONG");
            result = await Geoprocessing.ExecuteToolAsync("AddField_management", mva, null, cts.Token, null, GPExecuteToolFlags.None);
            //calculate field value with editor callback.
            await QueuedTask.Run(() =>
            {
                using (Geodatabase geodatabase = new Geodatabase(@"D:\Temp\Test.gdb"))
                using (FeatureClass featureClass = geodatabase.OpenDataset<FeatureClass>("Test"))
                using (FeatureClassDefinition featureClassDefinition = featureClass.GetDefinition())
                {
                    int someIndex = featureClassDefinition.FindField("someID");
                    EditOperation editOperation = new EditOperation();
                    editOperation.Callback(context =>
                    {
                        using (RowCursor rowCursor = featureClass.Search(null, false))
                        {
                            while (rowCursor.MoveNext())
                            {
                                using (Feature feature = (Feature)rowCursor.Current)
                                {
                                   
                                    feature[someIndex] = 42;
                                    feature.Store();
                                }
                            }
                        }
                    }, featureClass);
                    editOperation.ExecuteAsync();
                }
            });
            MessageBox.Show("Complete");
        }Cheers,
Luke