fix: Don't rewrap setters on the prototype (#4563)

Co-authored-by: Michel Weststrate <mweststrate@meta.com>

Author: mweststrate

.changeset/fair-houses-rhyme.md

@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+Fixed memory leak where makeAutoObservable would keep wrapping setters defined on the prototype. Fixes #4553

packages/mobx/src/types/autoannotation.ts

@@ -10,7 +10,8 @@ import {
     autoAction,
     isGenerator,
     MakeResult,
-    die
+    die,
+    isAction
 } from "../internal"
 
 const AUTO = "true"
@@ -40,7 +41,9 @@ function make_(
     // lone setter -> action setter
     if (descriptor.set) {
         // TODO make action applicable to setter and delegate to action.make_
-        const set = createAction(key.toString(), descriptor.set) as (v: any) => void
+        const set = isAction(descriptor.set)
+            ? descriptor.set // See #4553
+            : (createAction(key.toString(), descriptor.set) as (v: any) => void)
         // own
         if (source === adm.target_) {
             return adm.defineProperty_(key, {