diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 0ebc6802ac..1f31b53786 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -252,7 +252,7 @@ class SyncResult:
     presence: list[UserPresenceState]
     account_data: list[JsonDict]
     # user ID -> {profile field -> value | null if unset }
-    profile_updates: dict[str, dict[str, JsonValue | None]]
+    profile_updates: dict[str, dict[str, JsonValue | None] | None]
     joined: list[JoinedSyncResult]
     invited: list[InvitedSyncResult]
     knocked: list[KnockedSyncResult]
@@ -2223,7 +2223,7 @@ class SyncHandler:
         )
 
         # Serialise the profile updates into the sync response format.
-        profile_updates: dict[str, dict[str, JsonValue | None]] = {}
+        profile_updates: dict[str, dict[str, JsonValue | None] | None] = {}
         for other_user_id, fields in user_fields.items():
             profile_data = profile_data_by_user.get(other_user_id)
             if profile_data is None:
@@ -2324,7 +2324,7 @@ class SyncHandler:
 
         # Serialise the profile updates into the sync response format.
         # user ID -> {profile field -> value | null if unset }
-        profile_updates: dict[str, dict[str, JsonValue | None]] = {}
+        profile_updates: dict[str, dict[str, JsonValue | None] | None] = {}
 
         # Process field updates and users who have events in the sync response
         if users:
@@ -2351,7 +2351,7 @@ class SyncHandler:
                     # No profile data for this user, just return a blank dictionary
                     # in incremental sync, telling the clients to remove all profile
                     # information for this user.
-                    profile_updates[other_user_id] = {}
+                    profile_updates[other_user_id] = None
                     continue
 
                 per_user_updates: dict[str, JsonValue] = {}
@@ -2401,7 +2401,7 @@ class SyncHandler:
         if left_room_user_ids:
             for other_user_id in left_room_user_ids:
                 # Return an empty dictionary to the client
-                profile_updates[other_user_id] = {}
+                profile_updates[other_user_id] = None
 
         if profile_updates:
             sync_result_builder.profile_updates = profile_updates
@@ -3441,7 +3441,7 @@ class SyncResultBuilder:
 
     presence: list[UserPresenceState] = attr.Factory(list)
     account_data: list[JsonDict] = attr.Factory(list)
-    profile_updates: dict[str, dict[str, JsonValue | None]] = attr.Factory(dict)
+    profile_updates: dict[str, dict[str, JsonValue | None] | None] = attr.Factory(dict)
     joined: list[JoinedSyncResult] = attr.Factory(list)
     invited: list[InvitedSyncResult] = attr.Factory(list)
     knocked: list[KnockedSyncResult] = attr.Factory(list)
diff --git a/tests/handlers/test_sync.py b/tests/handlers/test_sync.py
index 7e3d5c7aa5..fbec22ab9b 100644
--- a/tests/handlers/test_sync.py
+++ b/tests/handlers/test_sync.py
@@ -1245,6 +1245,7 @@ class SyncProfileUpdatesTestCase(tests.unittest.HomeserverTestCase):
                 request_key=generate_request_key(),
             )
         )
+        assert initial_result.profile_updates["@other_user:test"] is not None
         self.assertEqual(
             initial_result.profile_updates["@other_user:test"]["m.status"],
             '{"text": "On holiday", "emoji": "\\ud83c\\udfd6"}',
@@ -1379,6 +1380,7 @@ class SyncProfileUpdatesTestCase(tests.unittest.HomeserverTestCase):
                 request_key=generate_request_key(),
             )
         )
+        assert incremental_result.profile_updates["@other_user:test"] is not None
         self.assertEqual(
             incremental_result.profile_updates["@other_user:test"]["m.status"],
             '{"text": "On holiday", "emoji": "\\ud83c\\udfd6"}',
@@ -1472,7 +1474,8 @@ class SyncProfileUpdatesTestCase(tests.unittest.HomeserverTestCase):
                 "@third_user:test",
             ],
         )
-        # This is a field upfate, so should be here
+        assert incremental_result.profile_updates["@other_user:test"] is not None
+        # This is a field update, so should be here
         self.assertEqual(
             incremental_result.profile_updates["@other_user:test"]["m.status"],
             '{"text": "On holiday", "emoji": "\\ud83c\\udfd6"}',
@@ -1482,6 +1485,7 @@ class SyncProfileUpdatesTestCase(tests.unittest.HomeserverTestCase):
         self.assertIsNone(
             incremental_result.profile_updates["@other_user:test"].get("displayname"),
         )
+        assert incremental_result.profile_updates["@third_user:test"] is not None
         # This user has events in the timeline, thus their full profile is included
         self.assertEqual(
             incremental_result.profile_updates["@third_user:test"]["m.status"],
@@ -1583,6 +1587,53 @@ class SyncProfileUpdatesTestCase(tests.unittest.HomeserverTestCase):
             [],
         )
 
+    @override_config({"experimental_features": {"msc4429_enabled": True}})
+    def test_incremental_sync_sends_down_null_profile_if_user_no_longer_sharing_rooms(
+        self,
+    ) -> None:
+        requester = create_requester(self.user)
+        initial_result = self.get_success(
+            self.sync_handler.wait_for_sync_for_user(
+                requester,
+                sync_config=generate_sync_config(
+                    user_id=self.user,
+                    filter_collection=FilterCollection(
+                        hs=self.hs,
+                        filter_json={
+                            "org.matrix.msc4429.profile_fields": {
+                                "ids": ["m.status", "displayname", "avatar_url"]
+                            }
+                        },
+                    ),
+                ),
+                request_key=generate_request_key(),
+            )
+        )
+        self.helper.leave(
+            room=self.joined_room, user=self.other_user, tok=self.other_tok
+        )
+        incremental_result = self.get_success(
+            self.sync_handler.wait_for_sync_for_user(
+                requester,
+                since_token=initial_result.next_batch,
+                sync_config=generate_sync_config(
+                    user_id=self.user,
+                    filter_collection=FilterCollection(
+                        hs=self.hs,
+                        filter_json={
+                            "org.matrix.msc4429.profile_fields": {
+                                "ids": ["m.status", "displayname", "avatar_url"]
+                            }
+                        },
+                    ),
+                ),
+                request_key=generate_request_key(),
+            )
+        )
+        self.assertIsNone(
+            incremental_result.profile_updates["@other_user:test"],
+        )
+
 
 class SyncStateAfterTestCase(tests.unittest.HomeserverTestCase):
     """Tests Sync Handler state behavior when using `use_state_after."""