ng-change not getting called on checkbox

I have a select dropdown. Upon selecting each value in dropdown, checkboxes are created using ng-repeat. All the values in the dropdown are supposed to be selected and corresponding checkboxes are stored in an array when a checkbox is checked using ng-change on the checkbox.

When I change back to a previous dropdown value, I am checking if the checkboxes are already there in the array used to store checked values and then check them using ng-check on load.

But when I uncheck the loaded checkboxes, SOMETIMES the ng-change is not called. I don't understand why.

When I check and uncheck it again, then it works.

Storing and removing checked values on checkbox change:

    $scope.checkChanged = function(isChecked, flat){
        //debugger;
        console.log("check change");
        if(isChecked != -1){
            let tempArray = [];
            for(let i=0; i<$scope.userEventData.selectedFlats.length; i++){
                tempArray.push($scope.userEventData.selectedFlats[i].flat_id);
            }
            if(!tempArray.includes(flat.flat_id)){
                $scope.userEventData.selectedFlats.push(flat);
                console.log("pushed new", flat.flat_no);
            }
        }else{
            $scope.userEventData.selectedFlats = $scope.userEventData.selectedFlats.filter(function(f){
                console.log(f, flat.flat_id);
                return f.flat_id != flat.flat_id
            });
            console.log("removed", flat.flat_no);
        }
        console.log($scope.userEventData.selectedFlats, "pushed check");
}

$scope.flatsArray is the array used to generate checkboxes using ng-repeat. Set checkbox to checked or unchecked based on array storing checked values($scope.userEventData.selectedFlats):

.then(function successCallback(response){ 
        $scope.flatsArray = response.data.body.Data;

        for(let d=0; d< $scope.flatsArray.length; d++){

            $scope.flatsArray[d].isChecked = false;

        }

        if($scope.userEventData.selectedFlats.length){
            let tempSelectedFlat = [];
            for(let j=0; j<$scope.userEventData.selectedFlats.length; j++){
                tempSelectedFlat.push($scope.userEventData.selectedFlats[j].flat_id);
            }
            console.log(tempSelectedFlat,"tempSelectedFlat");

            /** If present in selectedFlats set ng-checked to true **/
            for(let d=0; d< $scope.flatsArray.length; d++){
                console.log(d, "d");
                console.log($scope.flatsArray[d].flat_id, "$scope.flatsArray[d].flat_id");
                console.log(tempSelectedFlat.includes($scope.flatsArray[d].flat_id));
                if(tempSelectedFlat.includes($scope.flatsArray[d].flat_id)){
                    $scope.flatsArray[d].isChecked = true;
                }else{
                    $scope.flatsArray[d].isChecked = false;
                }

            }
        }else{
            for(let d=0; d< $scope.flatsArray.length; d++){

                $scope.flatsArray[d].isChecked = false;

            }
        }

The html:

    <div class="ScrollStyle">
   <div ng-repeat="flat in flatsArray" class="row" style="width:90%;">
      <div class="col-md-2">
         <input class="" name="{{flat.flat_no}}" 
            type="checkbox" ng-true-value="{{flat.flat_id}}" 
            ng-false-value="'-1'" 
            ng-checked="flat.isChecked"
            ng-change="checkChanged(sFlats[$index], flat)" ng-model="sFlats[$index]" />
      </div>
      <div class="col-md-10"><label for="{{flat.flat_no}}">{{flat.flat_no}}</label></div>
   </div>
</div>

$scope.flatsArray:

   0: {flat_id: "9", flat_no: "1", isChecked: false, $$hashKey: "object:62"}
1
:
{flat_id: "10", flat_no: "2", isChecked: false, $$hashKey: "object:63"}