Why doesn't the draggable index change?

Question

Vue.js

There is such a code, the essence is very simple. When dragging elements, the index does not change.
Perhaps I don’t understand how it works, I will be grateful for help
I expect to see the index change in task.sort, but I don’t see

<draggable
                                v-model="tasks"
                                :move="checkMove" >
                            <transition-group >
                                <div v-for="task in tasks" :key="task.sort" v-model="tasks">
                                    <div class="item" >
                                        {{task.name}}  {{task.sort}}
                                    </div>
                                </div>
                            </transition-group>
                        </draggable>

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