Mongoose update function is only saves doc changes half of the time - node.js

Question

The route below first runs a middleware which gives me access to req.user so I can get that users cart_id. Then I get the item which is being added to the cart through req.body (an item is uniquely identified by its id and color). Then I get the cart from the database and try to update it.
If the findIndex() function returns any number but -1, it means the item already is in the cart and I want to increase its quantity. If the function did not find the item, then I want to add it to the cart.
The strange problem I am getting is that when I try adding a new item (the code runs the else block where I push a new item to the cart) everything works and the changes are being saved. HOWEVER, when I try to add an item which is already in the cart, the changes are NOT being saved.
I have tried to console.log the quantity both before and after updating it and it is being updated correctly, but for some reason the changes aren't being saved to the database. Please help.
router.get("/add-to-cart", get_user, async (req, res) => {
try {
const { item, color, quantity } = req.body;
await Cart.findById(req.user.cart_id).then(doc => {
const i = doc.line_items.findIndex(elm => { return (elm.item == item && elm.color == color) });
if (i !== -1) {
console.log(doc.line_items[i].quantity) // 1
doc.line_items[i].quantity += quantity;
console.log(doc.line_items[i].quantity) // 2, but not being saved to database
}
else {
doc.line_items.push({ item, color, quantity }); // does get saved to database
}
doc.save();
}).catch(err => {
throw err.message;
});
res.json({ success: true });
}
catch (err) {
console.log(err);
res.status(500).json({ error: "unknown error" });
}
});
// the front end:
await fetch('/cart/add-to-cart', {
method: 'POST',
headers: { 'content-type': 'application/json' },
body: JSON.stringify({ item: 101, color: 'white', quantity: 1 })
}).then(res => res.json())
// the cart schema
{
_id: 123456,
line_items: [
{ item: 101, color: 'white', quantity: 1 },
{ item: 101, color: 'green', quantity: 1 },
{ item: 102, color: 'white', quantity: 1 },
]
}