diff --git a/public/packets.js b/public/packets.js
index b8fab5bf..e42bf384 100644
--- a/public/packets.js
+++ b/public/packets.js
@@ -357,7 +357,7 @@
           if (pktTime && pktTime < cutoff) return false;
         }
         if (filters.type) { const types = filters.type.split(',').map(Number); if (!types.includes(p.payload_type)) return false; }
-        if (filters.observer) { const obsSet = new Set(filters.observer.split(',')); if (!obsSet.has(p.observer_id)) return false; }
+        if (filters.observer) { const obsSet = new Set(filters.observer.split(',')); if (!obsSet.has(p.observer_id) && !(p._children && p._children.some(c => obsSet.has(String(c.observer_id))))) return false; }
         if (filters.hash && p.hash !== filters.hash) return false;
         if (RegionFilter.getRegionParam()) {
           const selectedRegions = RegionFilter.getRegionParam().split(',');
@@ -1290,9 +1290,13 @@
       const types = filters.type.split(',').map(Number);
       displayPackets = displayPackets.filter(p => types.includes(p.payload_type));
     }
-    if (filters.observer && !groupByHash) {
+    if (filters.observer) {
       const obsIds = new Set(filters.observer.split(','));
-      displayPackets = displayPackets.filter(p => obsIds.has(p.observer_id));
+      displayPackets = displayPackets.filter(p => {
+        if (obsIds.has(p.observer_id)) return true;
+        if (p._children) return p._children.some(c => obsIds.has(String(c.observer_id)));
+        return false;
+      });
     }
 
     // Packet Filter Language
diff --git a/test-frontend-helpers.js b/test-frontend-helpers.js
index ae3fce6d..b0ba546e 100644
--- a/test-frontend-helpers.js
+++ b/test-frontend-helpers.js
@@ -2695,6 +2695,63 @@ console.log('\n=== packets.js: savedTimeWindowMin defaults ===');
       'buildGroupRowHtml should use hoisted _observerFilterSet');
   });
 
+  test('observer filter in grouped mode includes packet when child matches (#537)', () => {
+    // The display filter should keep a grouped packet whose primary observer_id
+    // does NOT match, but one of its _children does.
+    const obsIds = new Set(['OBS_B']);
+    const packets = [
+      { observer_id: 'OBS_A', _children: [{ observer_id: 'OBS_A' }, { observer_id: 'OBS_B' }] },
+      { observer_id: 'OBS_C', _children: [{ observer_id: 'OBS_C' }] },
+    ];
+    const result = packets.filter(p => {
+      if (obsIds.has(p.observer_id)) return true;
+      if (p._children) return p._children.some(c => obsIds.has(String(c.observer_id)));
+      return false;
+    });
+    assert.strictEqual(result.length, 1, 'should keep packet with matching child observer');
+    assert.strictEqual(result[0].observer_id, 'OBS_A');
+  });
+
+  test('observer filter in grouped mode hides packet with no matching observations (#537)', () => {
+    const obsIds = new Set(['OBS_X']);
+    const packets = [
+      { observer_id: 'OBS_A', _children: [{ observer_id: 'OBS_A' }, { observer_id: 'OBS_B' }] },
+    ];
+    const result = packets.filter(p => {
+      if (obsIds.has(p.observer_id)) return true;
+      if (p._children) return p._children.some(c => obsIds.has(String(c.observer_id)));
+      return false;
+    });
+    assert.strictEqual(result.length, 0, 'should hide packet with no matching observers');
+  });
+
+  test('WS observer filter checks children for grouped packets (#537)', () => {
+    const filters = { observer: 'OBS_B' };
+    const obsSet = new Set(filters.observer.split(','));
+    const p = { observer_id: 'OBS_A', _children: [{ observer_id: 'OBS_B' }] };
+    const passes = obsSet.has(p.observer_id) || (p._children && p._children.some(c => obsSet.has(String(c.observer_id))));
+    assert.ok(passes, 'WS filter should pass grouped packet when child matches');
+
+    const p2 = { observer_id: 'OBS_C', _children: [{ observer_id: 'OBS_D' }] };
+    const passes2 = obsSet.has(p2.observer_id) || (p2._children && p2._children.some(c => obsSet.has(String(c.observer_id))));
+    assert.ok(!passes2, 'WS filter should reject grouped packet with no matching observers');
+  });
+
+  test('packets.js display filter checks _children for observer match (#537)', () => {
+    // Verify the actual source code has the children check
+    assert.ok(
+      packetsSource.includes('p._children) return p._children.some(c => obsIds.has(String(c.observer_id))'),
+      'display filter should check _children for observer match'
+    );
+  });
+
+  test('packets.js WS filter checks _children for observer match (#537)', () => {
+    assert.ok(
+      packetsSource.includes('p._children && p._children.some(c => obsSet.has(String(c.observer_id)))'),
+      'WS filter should check _children for observer match'
+    );
+  });
+
   test('buildFlatRowHtml has null-safe decoded_json', () => {
     const flatBuilderMatch = packetsSource.match(/function buildFlatRowHtml[\s\S]*?(?=\n  function )/);
     assert.ok(flatBuilderMatch, 'buildFlatRowHtml should exist');