Redux reducer is not returning entire state

I’m new to redux,My problem is when i dispatch action, it is going to the reducer but the state is not updating.

This is my initial state

   const initialState ={
    cart :[ ],
    Products:[
        {
            id:"1",
            title:"Realme 3 pro",
            image:{
                img1:"https://www.monotain.com/wp-content/uploads/2019/12/realme-5-pro.jpg",
                img2:"https://static.techspot.com/images/products/2019/smartphones/org/2019-05-20-product-7.jpg"
            },
           
            price:"10000",
            details:"Take photography a notch higher with the Realme 3 Pro as it comes with a 25 MP AI Selfie Camera and 16 MP + 5 MP Rear Camera with Sony IMX519 Sensor. The Qualcomm Snapdragon 710 AIE, multi-core AI Engine and Adreno 616 GPU redefine your smartphone experience.",
            seller:"Cloudtail India",
            quantity:"0"
    
    
        }
       
       
    
     ]
     
}

** This is my reducer which is handling this action **

         case UPDATE_QUANTITY:
              
                return{
                    ...state,
                    cart:state.cart.map((product,i) =>
                    {
                        if(i === (action.product.id-1))
                       {
                           product.quantity = action.qty.toString();
                         

                       }
                       
                        
                    })
                } 

Other reducers are working fine and returning correct state but when i dispatch this action,My state is not updating .If there is any improvement to be done on code .Please specify