fix(Watermark): ensure watermark updates on imageBase64 change (#13474)

packages/vant/src/watermark/Watermark.tsx

@@ -1,5 +1,6 @@
 import {
   defineComponent,
+  nextTick,
   onMounted,
   onUnmounted,
   ref,
@@ -158,7 +159,6 @@ export default defineComponent({
 
     watch(
       () => [
-        imageBase64.value,
         props.content,
         props.textColor,
         props.height,
@@ -170,6 +170,10 @@ export default defineComponent({
       generateWatermarkUrl,
     );
 
+    watch(imageBase64, () => {
+      nextTick(generateWatermarkUrl);
+    });
+
     onMounted(generateWatermarkUrl);
 
     onUnmounted(revokeWatermarkUrl);