diff --git a/public/packets.js b/public/packets.js
index 8c5be8b9..a059e7c6 100644
--- a/public/packets.js
+++ b/public/packets.js
@@ -2026,7 +2026,8 @@
       buildObserverMenu();
       updateObsTrigger();
       updatePacketsUrl();
-      renderTableRows();
+      // Grouped rows are filtered by the server only (#1821), so re-fetch.
+      loadPackets();
     });
 
     // --- Type multi-select ---
diff --git a/tests/unit/test-observer-menu-interactions.js b/tests/unit/test-observer-menu-interactions.js
index 7696685a..96becc95 100644
--- a/tests/unit/test-observer-menu-interactions.js
+++ b/tests/unit/test-observer-menu-interactions.js
@@ -63,12 +63,13 @@ function stubEl(overrides) {
     const updateObsTrigger = () => calls.push('updateObsTrigger');
     const updatePacketsUrl = () => calls.push('updatePacketsUrl');
     const renderTableRows = () => calls.push('renderTableRows');
+    const loadPackets = () => calls.push('loadPackets');
     const fn = new Function(
       'e', 'selectedObservers', 'filters', 'localStorage',
-      'buildObserverMenu', 'updateObsTrigger', 'updatePacketsUrl', 'renderTableRows',
+      'buildObserverMenu', 'updateObsTrigger', 'updatePacketsUrl', 'renderTableRows', 'loadPackets',
       body
     );
-    fn(target, selectedObservers, filters, localStorage, buildObserverMenu, updateObsTrigger, updatePacketsUrl, renderTableRows);
+    fn(target, selectedObservers, filters, localStorage, buildObserverMenu, updateObsTrigger, updatePacketsUrl, renderTableRows, loadPackets);
   }
 
   test('change event with no data-obs-id (bubbled from search input) is a no-op', () => {
@@ -88,6 +89,24 @@ function stubEl(overrides) {
     assert(calls.includes('buildObserverMenu'), 'buildObserverMenu should run for a real toggle');
     assert(calls.includes('updatePacketsUrl'), 'updatePacketsUrl should run for a real toggle');
   });
+
+  // Grouped mode leaves the observer filter to the server (#1821), so a
+  // re-render of the already loaded rows changes nothing until a reload.
+  test('toggling an observer re-fetches packets from the server', () => {
+    const calls = [];
+    const filters = {};
+    run({ target: { dataset: { obsId: '7' }, checked: true } }, { calls, selectedObservers: new Set(), filters });
+    assert.strictEqual(filters.observer, '7');
+    assert(calls.includes('loadPackets'), 'loadPackets must run so the server applies the observer filter');
+  });
+
+  test('selecting All Observers re-fetches packets from the server', () => {
+    const calls = [];
+    const filters = { observer: '7' };
+    run({ target: { dataset: { obsId: '__all__' }, checked: true } }, { calls, selectedObservers: new Set(['7']), filters });
+    assert.strictEqual(filters.observer, undefined);
+    assert(calls.includes('loadPackets'), 'loadPackets must run so the unfiltered set is fetched');
+  });
 }
 
 // --- 2. Clear Filters must reset and re-apply the observer search box ---