diff --git a/changelog.d/6819.misc b/changelog.d/6819.misc
new file mode 100644
index 000000000..4f9a4ac7a
--- /dev/null
+++ b/changelog.d/6819.misc
@@ -0,0 +1 @@
+Detect unknown remote devices and mark cache as stale.
diff --git a/synapse/storage/data_stores/main/devices.py b/synapse/storage/data_stores/main/devices.py
index a34415ff1..ea0503476 100644
--- a/synapse/storage/data_stores/main/devices.py
+++ b/synapse/storage/data_stores/main/devices.py
@@ -940,13 +940,6 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
             lock=False,
         )
 
-        # If we're replacing the remote user's device list cache presumably
-        # we've done a full resync, so we remove the entry that says we need
-        # to resync
-        self.db.simple_delete_txn(
-            txn, table="device_lists_remote_resync", keyvalues={"user_id": user_id},
-        )
-
     def update_remote_device_list_cache(self, user_id, devices, stream_id):
         """Replace the entire cache of the remote user's devices.
 
@@ -1003,6 +996,13 @@ class DeviceStore(DeviceWorkerStore, DeviceBackgroundUpdateStore):
             lock=False,
         )
 
+        # If we're replacing the remote user's device list cache presumably
+        # we've done a full resync, so we remove the entry that says we need
+        # to resync
+        self.db.simple_delete_txn(
+            txn, table="device_lists_remote_resync", keyvalues={"user_id": user_id},
+        )
+
     @defer.inlineCallbacks
     def add_device_change_to_streams(self, user_id, device_ids, hosts):
         """Persist that a user's devices have been updated, and which hosts