fixes

packages/svelte/src/internal/client/runtime.js

@@ -1432,6 +1432,8 @@ export function prop(props, key, flags, initial) {
 		}
 
 		value = /** @type {V} */ (initial);
+
+		if (setter) setter(value);
 	}
 
 	var getter = () => {

packages/svelte/tests/runtime-legacy/samples/binding-backflow/Child.svelte

@@ -16,5 +16,5 @@
 	export let updates = [];
 	$: updates = [...updates, value];
 </script>
-  
+
 <div>child: {value?.foo} | updates: {updates.length}</div>

packages/svelte/tests/runtime-legacy/samples/binding-backflow/_config.js

@@ -34,7 +34,7 @@ export default test({
 
 		p = parents['reactive_mutate'];
 		assert.deepEqual(p.value, { foo: 'kid' });
-		assert.equal(p.updates.length, 2);
+		assert.equal(p.updates.length, 1);
 
 		p = parents['init_update'];
 		assert.deepEqual(p.value, { foo: 'kid' });
@@ -42,6 +42,6 @@ export default test({
 
 		p = parents['init_mutate'];
 		assert.deepEqual(p.value, { foo: 'kid' });
-		assert.equal(p.updates.length, 2);
+		assert.equal(p.updates.length, 1);
 	}
 });