fix(watch): fix flush: pre watchers triggered synchronously in setup

fix #5721

packages/runtime-core/src/apiWatch.ts

@@ -345,15 +345,7 @@ function doWatch(
     scheduler = () => queuePostRenderEffect(job, instance && instance.suspense)
   } else {
     // default: 'pre'
-    scheduler = () => {
-      if (!instance || instance.isMounted) {
-        queuePreFlushCb(job)
-      } else {
-        // with 'pre' option, the first call must happen before
-        // the component is mounted so it is called synchronously.
-        job()
-      }
-    }
+    scheduler = () => queuePreFlushCb(job)
   }
 
   const effect = new ReactiveEffect(getter, scheduler)

packages/runtime-core/src/scheduler.ts

@@ -182,6 +182,8 @@ export function flushPreFlushCbs(
 }
 
 export function flushPostFlushCbs(seen?: CountMap) {
+  // flush any pre cbs queued during the flush (e.g. pre watchers)
+  flushPreFlushCbs()
   if (pendingPostFlushCbs.length) {
     const deduped = [...new Set(pendingPostFlushCbs)]
     pendingPostFlushCbs.length = 0