fix(runtime-core): ensure watchers are always registered to correct instance owner (#2495)

close: #2381

packages/runtime-core/src/apiWatch.ts

@@ -285,7 +285,7 @@ function doWatch(
     scheduler
   })
 
-  recordInstanceBoundEffect(runner)
+  recordInstanceBoundEffect(runner, instance)
 
   // initial run
   if (cb) {

packages/runtime-core/src/component.ts

@@ -786,9 +786,12 @@ export function createSetupContext(
 
 // record effects created during a component's setup() so that they can be
 // stopped when the component unmounts
-export function recordInstanceBoundEffect(effect: ReactiveEffect) {
-  if (currentInstance) {
-    ;(currentInstance.effects || (currentInstance.effects = [])).push(effect)
+export function recordInstanceBoundEffect(
+  effect: ReactiveEffect,
+  instance = currentInstance
+) {
+  if (instance) {
+    ;(instance.effects || (instance.effects = [])).push(effect)
   }
 }