diff --git a/playwright/widget/federation-oldest-membership-bug.spec.ts b/playwright/widget/federation-oldest-membership-bug.spec.ts
new file mode 100644
index 00000000..70442e05
--- /dev/null
+++ b/playwright/widget/federation-oldest-membership-bug.spec.ts
@@ -0,0 +1,92 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { expect, test } from "@playwright/test";
+
+import { widgetTest } from "../fixtures/widget-user";
+import { HOST1, HOST2, TestHelpers } from "./test-helpers";
+
+widgetTest(
+  "Bug new joiner was not publishing on correct SFU",
+  async ({ addUser, browserName }) => {
+    test.skip(
+      browserName === "firefox",
+      "This is a bug in the old widget, not a browser problem.",
+    );
+
+    test.slow();
+
+    // 2 users in federation
+    const florian = await addUser("floriant", HOST1);
+    const timo = await addUser("timo", HOST2);
+
+    // Florian creates a room and invites Timo to it
+    const roomName = "Call Room";
+    await TestHelpers.createRoom(roomName, florian.page, [timo.mxId]);
+
+    // Timo joins the room
+    await TestHelpers.acceptRoomInvite(roomName, timo.page);
+
+    // Ensure we are in legacy mode (should be the default)
+    await TestHelpers.openWidgetSetEmbeddedElementCallRtcModeCloseWidget(
+      florian.page,
+      "legacy",
+    );
+    await TestHelpers.openWidgetSetEmbeddedElementCallRtcModeCloseWidget(
+      timo.page,
+      "legacy",
+    );
+
+    // Let timo create a call
+    await TestHelpers.startCallInCurrentRoom(timo.page, false);
+    await TestHelpers.joinCallFromLobby(timo.page);
+
+    // We want to simulate that the oldest membership authentication is way slower than
+    // the preffered auth.
+    // In this setup, timo advertised$ transport will be it's own, and the active will be the one from florian
+    await florian.page.route(
+      "**/matrix-rtc.othersite.m.localhost/livekit/jwt/**",
+      async (route) => {
+        await new Promise((resolve) => setTimeout(resolve, 2000)); // 5 second delay
+        await route.continue();
+      },
+    );
+
+    // Florian joins the call
+    await expect(florian.page.getByTestId("join-call-button")).toBeVisible();
+    await florian.page.getByTestId("join-call-button").click();
+    await TestHelpers.joinCallFromLobby(florian.page);
+
+    await florian.page.waitForTimeout(3000);
+    await timo.page.waitForTimeout(3000);
+
+    // We should see 2 video tiles everywhere now
+    for (const user of [timo, florian]) {
+      const frame = user.page
+        .locator('iframe[title="Element Call"]')
+        .contentFrame();
+      await expect(frame.getByTestId("videoTile")).toHaveCount(2);
+
+      // No one should be waiting for media
+      await expect(frame.getByText("Waiting for media...")).not.toBeVisible();
+
+      // There should be 2 video elements, visible and autoplaying
+      const videoElements = await frame.locator("video").all();
+      expect(videoElements.length).toBe(2);
+
+      const blockDisplayCount = await frame
+        .locator("video")
+        .evaluateAll(
+          (videos: Element[]) =>
+            videos.filter(
+              (v: Element) => window.getComputedStyle(v).display === "block",
+            ).length,
+        );
+      expect(blockDisplayCount).toBe(2);
+    }
+  },
+);