diff --git a/packages/@ember/-internals/metal/lib/tags.ts b/packages/@ember/-internals/metal/lib/tags.ts
index 43bebc531de..a8ce2feb1a9 100644
--- a/packages/@ember/-internals/metal/lib/tags.ts
+++ b/packages/@ember/-internals/metal/lib/tags.ts
@@ -7,7 +7,6 @@ import {
   toString,
 } from '@ember/-internals/utils';
 import { assert, deprecate } from '@ember/debug';
-import { _WeakSet as WeakSet } from '@ember/polyfills';
 import { backburner } from '@ember/runloop';
 import { DEBUG } from '@glimmer/env';
 import {
@@ -55,12 +54,6 @@ export const UNKNOWN_PROPERTY_TAG = symbol('UNKNOWN_PROPERTY_TAG');
 // This is exported for `@tracked`, but should otherwise be avoided. Use `tagForObject`.
 export const SELF_TAG: string = symbol('SELF_TAG');
 
-let SEEN_TAGS: WeakSet<Tag> | undefined;
-
-if (DEBUG) {
-  SEEN_TAGS = new WeakSet();
-}
-
 export function tagForProperty(obj: unknown, propertyKey: string | symbol): Tag {
   if (!isObject(obj)) {
     return CONSTANT_TAG;
@@ -72,11 +65,10 @@ export function tagForProperty(obj: unknown, propertyKey: string | symbol): Tag
 
   let tag = tagFor(obj, propertyKey);
 
-  if (DEBUG && !SEEN_TAGS!.has(tag)) {
-    SEEN_TAGS!.add(tag);
-
-    setupMandatorySetter!(obj, propertyKey);
+  if (DEBUG) {
+    setupMandatorySetter!(tag, obj, propertyKey);
 
+    // TODO: Replace this with something more first class for tracking tags in DEBUG
     (tag as any)._propertyKey = propertyKey;
   }
 
diff --git a/packages/@ember/-internals/runtime/lib/mixins/-proxy.js b/packages/@ember/-internals/runtime/lib/mixins/-proxy.js
index 57c66f7c60b..8f61ea63343 100644
--- a/packages/@ember/-internals/runtime/lib/mixins/-proxy.js
+++ b/packages/@ember/-internals/runtime/lib/mixins/-proxy.js
@@ -10,12 +10,13 @@ import {
   Mixin,
   tagForObject,
   computed,
-  UNKNOWN_PROPERTY_TAG,
   getChainTagsForKey,
+  UNKNOWN_PROPERTY_TAG,
 } from '@ember/-internals/metal';
-import { setProxy } from '@ember/-internals/utils';
+import { setProxy, setupMandatorySetter } from '@ember/-internals/utils';
 import { assert } from '@ember/debug';
-import { combine, update } from '@glimmer/validator';
+import { DEBUG } from '@glimmer/env';
+import { combine, update, tagFor } from '@glimmer/validator';
 
 export function contentFor(proxy) {
   let content = get(proxy, 'content');
@@ -58,7 +59,16 @@ export default Mixin.create({
   }),
 
   [UNKNOWN_PROPERTY_TAG](key) {
-    return combine(getChainTagsForKey(this, `content.${key}`));
+    let tag = tagFor(this, key);
+
+    if (DEBUG) {
+      setupMandatorySetter(tag, this, key);
+
+      // TODO: Replace this with something more first class for tracking tags in DEBUG
+      tag._propertyKey = key;
+    }
+
+    return combine([tag, ...getChainTagsForKey(this, `content.${key}`)]);
   },
 
   unknownProperty(key) {
diff --git a/packages/@ember/-internals/runtime/tests/system/object_proxy_test.js b/packages/@ember/-internals/runtime/tests/system/object_proxy_test.js
index 9a2ecda3f62..c7c2d26179a 100644
--- a/packages/@ember/-internals/runtime/tests/system/object_proxy_test.js
+++ b/packages/@ember/-internals/runtime/tests/system/object_proxy_test.js
@@ -338,5 +338,33 @@ moduleFor(
         observe: observer('foo', function() {}),
       }).create();
     }
+
+    async '@test custom proxies should be able to notify property changes manually'(assert) {
+      let proxy = ObjectProxy.extend({
+        locals: { foo: 123 },
+
+        unknownProperty(key) {
+          return this.locals[key];
+        },
+
+        setUnknownProperty(key, value) {
+          this.locals[key] = value;
+          this.notifyPropertyChange(key);
+        },
+      }).create();
+
+      let count = 0;
+
+      proxy.addObserver('foo', function() {
+        count++;
+      });
+
+      proxy.set('foo', 456);
+      await runLoopSettled();
+
+      assert.equal(count, 1);
+      assert.equal(proxy.get('foo'), 456);
+      assert.equal(proxy.get('locals.foo'), 456);
+    }
   }
 );
diff --git a/packages/@ember/-internals/utils/lib/mandatory-setter.ts b/packages/@ember/-internals/utils/lib/mandatory-setter.ts
index c8dd855e49a..b7541877554 100644
--- a/packages/@ember/-internals/utils/lib/mandatory-setter.ts
+++ b/packages/@ember/-internals/utils/lib/mandatory-setter.ts
@@ -1,8 +1,10 @@
 import { assert } from '@ember/debug';
+import { _WeakSet as WeakSet } from '@ember/polyfills';
 import { DEBUG } from '@glimmer/env';
+import { Tag } from '@glimmer/validator';
 import lookupDescriptor from './lookup-descriptor';
 
-export let setupMandatorySetter: ((obj: object, keyName: string | symbol) => void) | undefined;
+export let setupMandatorySetter: ((tag: Tag, obj: object, keyName: string | symbol) => void) | undefined;
 export let teardownMandatorySetter: ((obj: object, keyName: string | symbol) => void) | undefined;
 export let setWithMandatorySetter:
   | ((obj: object, keyName: string | symbol, value: any) => void)
@@ -11,6 +13,8 @@ export let setWithMandatorySetter:
 type PropertyDescriptorWithMeta = PropertyDescriptor & { hadOwnProperty?: boolean };
 
 if (DEBUG) {
+  let SEEN_TAGS = new WeakSet();
+
   let MANDATORY_SETTERS: WeakMap<
     object,
     // @ts-ignore
@@ -21,7 +25,13 @@ if (DEBUG) {
     return Object.prototype.propertyIsEnumerable.call(obj, key);
   };
 
-  setupMandatorySetter = function(obj: object, keyName: string | symbol) {
+  setupMandatorySetter = function(tag: Tag, obj: object, keyName: string | symbol) {
+    if (SEEN_TAGS.has(tag)) {
+      return;
+    }
+
+    SEEN_TAGS!.add(tag);
+
     let desc = (lookupDescriptor(obj, keyName) as PropertyDescriptorWithMeta) || {};
 
     if (desc.get || desc.set) {