diff --git a/addons/mail/static/src/core/common/message.xml b/addons/mail/static/src/core/common/message.xml
index ade64face50..86fa237e177 100644
--- a/addons/mail/static/src/core/common/message.xml
+++ b/addons/mail/static/src/core/common/message.xml
@@ -18,7 +18,7 @@
                         <div t-if="!props.squashed" class="o-mail-Message-avatarContainer position-relative bg-view">
                             <img class="o-mail-Message-avatar w-100 h-100 rounded" t-att-src="authorAvatarUrl" t-att-class="authorAvatarAttClass"/>
                         </div>
-                        <t t-elif="!message.isTransient">
+                        <t t-elif="!message.is_transient">
                             <small t-if="isActive and props.showDates" class="o-mail-Message-date text-muted opacity-75">
                                 <t t-esc="messageService.dateSimple(message)"/>
                             </small>
diff --git a/addons/mail/static/src/core/common/thread_model.js b/addons/mail/static/src/core/common/thread_model.js
index ef368c0a41f..0a2f65ee90e 100644
--- a/addons/mail/static/src/core/common/thread_model.js
+++ b/addons/mail/static/src/core/common/thread_model.js
@@ -492,7 +492,7 @@ export class Thread extends Record {
     }
 
     get persistentMessages() {
-        return this.messages.filter((message) => !message.isTransient);
+        return this.messages.filter((message) => !message.is_transient);
     }
 
     get prefix() {
diff --git a/addons/mail/static/tests/message/message_tests.js b/addons/mail/static/tests/message/message_tests.js
index 4ec78263108..4555b9429e8 100644
--- a/addons/mail/static/tests/message/message_tests.js
+++ b/addons/mail/static/tests/message/message_tests.js
@@ -9,6 +9,8 @@ import { deserializeDateTime } from "@web/core/l10n/dates";
 import { getOrigin } from "@web/core/utils/urls";
 import {
     makeDeferred,
+    nextTick,
+    patchDate,
     patchTimeZone,
     patchWithCleanup,
     triggerHotkey,
@@ -672,6 +674,41 @@ QUnit.test("should not be able to reply to temporary/transient messages", async
     await contains(".o-mail-Message [title='Reply']", { count: 0 });
 });
 
+QUnit.test("squashed transient message should not have date in the sidebar", async () => {
+    patchDate(2024, 2, 26, 10, 0, 0);
+    const pyEnv = await startServer();
+    const channelId = pyEnv["discuss.channel"].create({ name: "Channel 1" });
+    pyEnv["mail.message"].create([
+        {
+            body: "Hello world 1",
+            model: "discuss.channel",
+            res_id: channelId,
+        },
+        {
+            body: "Hello world 2",
+            model: "discuss.channel",
+            res_id: channelId,
+        },
+    ]);
+    const { openDiscuss } = await start();
+    await openDiscuss(channelId);
+    await click(".o-mail-Message.o-squashed");
+    await contains(".o-mail-Message.o-squashed .o-mail-Message-sidebar", {
+        text: "10:00",
+    });
+    await insertText(".o-mail-Composer-input", "/who");
+    await click(".o-mail-Composer-send:enabled");
+    await contains(".o-mail-Message", { text: "You are alone in this channel." });
+    await insertText(".o-mail-Composer-input", "/who");
+    await click(".o-mail-Composer-send:enabled");
+    await click(":nth-child(2 of .o-mail-Message.o-squashed");
+    await nextTick();
+    await contains(":nth-child(2 of .o-mail-Message.o-squashed) .o-mail-Message-sidebar", {
+        text: "10:00",
+        count: 0,
+    });
+});
+
 QUnit.test("message comment of same author within 1min. should be squashed", async () => {
     // messages are squashed when "close", e.g. less than 1 minute has elapsed
     // from messages of same author and same thread. Note that this should