diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -1493,7 +1493,9 @@
         action.payload.messageStoreThreads ?? [],
       );
 
-    const newThreads = {};
+    const newThreads: {
+      [threadID: string]: ThreadMessageInfo,
+    } = { ...messageStore.threads };
     for (const threadID in actionPayloadMessageStoreThreads) {
       newThreads[threadID] = {
         ...actionPayloadMessageStoreThreads[threadID],