Because defineModel() only emits update:modelValue when you reassign model.value itself. Mutating a property inside the object (model.value.name = 'x') changes the object in place without changing the reference, so Vue never fires the update:modelValue event. Parent watchers on the v-model binding won't detect the change, though the parent's template still reflects the mutation because both sides share the same reactive object.

<script setup>
const model = defineModel<{ name: string; age: number }>()

function updateName(newName: string) {
  model.value.name = newName // parent never knows
}
</script>

Open in Vue Playground

How to fix it

Replace the entire object so the reference changes and the event fires.

function updateName(newName: string) {
  model.value = { ...model.value, name: newName }
}

For arrays, same idea:

function addItem(item: string) {
  model.value = { ...model.value, items: [...model.value.items, item] }
}

Deep nesting gets verbose

// Deeply nested update
model.value = {
  ...model.value,
  user: {
    ...model.value.user,
    address: { ...model.value.user.address, city: 'Madrid' }
  }
}

// structuredClone is cleaner for complex objects
function updateCity(city: string) {
  const updated = structuredClone(model.value)
  updated.user.address.city = city
  model.value = updated
}

Avoid unnecessary double emissions

When updating multiple fields, batch them into a single assignment. Two consecutive spreads both read the correct model.value (ref assignments are synchronous), but each one creates a new object reference, emitting update:modelValue twice. This is wasteful and may cause unnecessary parent re-renders.

// works, but emits update:modelValue twice
model.value = { ...model.value, a: '1' }
model.value = { ...model.value, b: '2' }

// better: single assignment, single emission
model.value = { ...model.value, a: '1', b: '2' }

See also: How does v-model work on custom components? · How do multiple v-model bindings work? · Why does mutating props cause warnings?

References

• defineModel() - Vue.js docs
• Component v-model - Vue.js docs