[BUGFIX] observed properties not being marked as enum

packages/@ember/-internals/metal/lib/property_set.ts

@@ -1,5 +1,5 @@
 import { descriptorFor, Meta, peekMeta } from '@ember/-internals/meta';
-import { HAS_NATIVE_PROXY, toString } from '@ember/-internals/utils';
+import { HAS_NATIVE_PROXY, lookupDescriptor, toString } from '@ember/-internals/utils';
 import { assert } from '@ember/debug';
 import EmberError from '@ember/error';
 import { DEBUG } from '@glimmer/env';
@@ -19,6 +19,7 @@ let setWithMandatorySetter: <T extends object, K extends Extract<keyof T, string
   keyName: K,
   value: T[K]
 ) => void;
+
 let makeEnumerable: (obj: object, keyName: string) => void;
 
 /**
@@ -125,9 +126,13 @@ if (DEBUG) {
   };
 
   makeEnumerable = (obj: object, key: string) => {
-    let desc = Object.getOwnPropertyDescriptor(obj, key);
+    let desc = lookupDescriptor(obj, key);
 
-    if (desc && desc.set && (desc.set as MandatorySetterFunction).isMandatorySetter) {
+    if (
+      desc !== null &&
+      desc.set !== undefined &&
+      (desc.set as MandatorySetterFunction).isMandatorySetter
+    ) {
       desc.enumerable = true;
       Object.defineProperty(obj, key, desc);
     }

packages/@ember/-internals/runtime/tests/system/core_object_test.js

@@ -1,5 +1,5 @@
 import { getOwner, setOwner } from '@ember/-internals/owner';
-import { get } from '@ember/-internals/metal';
+import { get, set, observer } from '@ember/-internals/metal';
 import CoreObject from '../../lib/system/core_object';
 import { moduleFor, AbstractTestCase, buildOwner } from 'internal-test-helpers';
 
@@ -106,5 +106,28 @@ moduleFor(
         },
       }).create(options);
     }
+
+    ['@test observed properties are enumerable when set GH#14594'](assert) {
+      let callCount = 0;
+      let Test = CoreObject.extend({
+        myProp: null,
+        anotherProp: undefined,
+        didChangeMyProp: observer('myProp', function() {
+          callCount++;
+        }),
+      });
+
+      let test = Test.create();
+      set(test, 'id', '3');
+      set(test, 'myProp', { id: 1 });
+
+      assert.deepEqual(Object.keys(test).sort(), ['id', 'myProp']);
+
+      set(test, 'anotherProp', 'nice');
+
+      assert.deepEqual(Object.keys(test).sort(), ['anotherProp', 'id', 'myProp']);
+
+      assert.equal(callCount, 1);
+    }
   }
 );