fix: revert props legacy mode regression

packages/svelte/src/internal/client/reactivity/props.js

@@ -336,7 +336,7 @@ export function prop(props, key, flags, fallback) {
 	}
 
 	// prop is never written to — we only need a getter
-	if ((flags & PROPS_IS_UPDATED) === 0) {
+	if (runes && (flags & PROPS_IS_UPDATED) === 0) {
 		return getter;
 	}
 
@@ -362,8 +362,10 @@ export function prop(props, key, flags, fallback) {
 		};
 	}
 
-	// prop is written to, but there's no binding, which means we
-	// create a derived that we can write to locally
+	// Either prop is written to, but there's no binding, which means we
+	// create a derived that we can write to locally.
+	// Or we are in legacy mode where we always create a derived to replicate that
+	// Svelte 4 did not trigger updates when a primitive value was updated to the same value.
 	var d = ((flags & PROPS_IS_IMMUTABLE) !== 0 ? derived : derived_safe_equal)(getter);
 
 	// Capture the initial value if it's bindable

packages/svelte/tests/runtime-legacy/samples/prop-no-change/_config.js

@@ -2,6 +2,7 @@ import { flushSync } from 'svelte';
 import { test } from '../../test';
 
 export default test({
+	accessors: false,
 	test({ assert, logs, target }) {
 		assert.deepEqual(logs, ['primitive', 'object']);
 		target.querySelector('button')?.click();