diff --git a/public/packet-helpers.js b/public/packet-helpers.js
index e75dee48..5a97a573 100644
--- a/public/packet-helpers.js
+++ b/public/packet-helpers.js
@@ -20,6 +20,17 @@ window.getParsedPath = function getParsedPath(p) {
   return p._parsedPath;
 };
 
+/**
+ * Clear cached _parsedPath/_parsedDecoded from a packet object.
+ * Must be called after spreading a parent packet into an observation/child,
+ * otherwise the child inherits stale cached values from the parent (issue #504).
+ */
+window.clearParsedCache = function clearParsedCache(p) {
+  delete p._parsedPath;
+  delete p._parsedDecoded;
+  return p;
+};
+
 window.getParsedDecoded = function getParsedDecoded(p) {
   if (p._parsedDecoded !== undefined) return p._parsedDecoded;
   var raw = p.decoded_json;
diff --git a/public/packets.js b/public/packets.js
index b7923599..5fccb69b 100644
--- a/public/packets.js
+++ b/public/packets.js
@@ -269,6 +269,7 @@
             if (obs) {
               expandedHashes.add(h);
               const obsPacket = {...data.packet, observer_id: obs.observer_id, observer_name: obs.observer_name, snr: obs.snr, rssi: obs.rssi, path_json: obs.path_json, timestamp: obs.timestamp, first_seen: obs.timestamp};
+              clearParsedCache(obsPacket);
               selectPacket(obs.id, h, {packet: obsPacket, breakdown: data.breakdown, observations: data.observations}, obs.id);
             } else {
               selectPacket(data.packet.id, h, data);
@@ -501,7 +502,7 @@
         await Promise.all(multiObs.map(async (p) => {
           try {
             const d = await api(`/packets/${p.hash}`);
-            if (d?.observations) p._children = d.observations.map(o => ({...d.packet, ...o, _isObservation: true}));
+            if (d?.observations) p._children = d.observations.map(o => clearParsedCache({...d.packet, ...o, _isObservation: true}));
           } catch {}
         }));
         // Flatten: replace grouped packets with individual observations
@@ -837,7 +838,7 @@
           try {
             const data = await api(`/packets/${p.hash}`);
             if (data?.packet && data.observations) {
-              p._children = data.observations.map(o => ({...data.packet, ...o, _isObservation: true}));
+              p._children = data.observations.map(o => clearParsedCache({...data.packet, ...o, _isObservation: true}));
               p._fetchedData = data;
             }
           } catch {}
@@ -1010,6 +1011,7 @@
           if (child) {
             const parentData = group._fetchedData;
             const obsPacket = parentData ? {...parentData.packet, observer_id: child.observer_id, observer_name: child.observer_name, snr: child.snr, rssi: child.rssi, path_json: child.path_json, timestamp: child.timestamp, first_seen: child.timestamp} : child;
+            if (parentData) { clearParsedCache(obsPacket); }
             selectPacket(child.id, parentHash, {packet: obsPacket, breakdown: parentData?.breakdown, observations: parentData?.observations}, child.id);
           }
         }
@@ -1979,7 +1981,7 @@
       if (!pkt) return;
       const group = packets.find(p => p.hash === hash);
       if (group && data.observations) {
-        group._children = data.observations.map(o => ({...pkt, ...o, _isObservation: true}));
+        group._children = data.observations.map(o => clearParsedCache({...pkt, ...o, _isObservation: true}));
         group._fetchedData = data;
         // Sort children based on current sort mode
         sortGroupChildren(group);
diff --git a/test-frontend-helpers.js b/test-frontend-helpers.js
index da850804..0fbbe6d5 100644
--- a/test-frontend-helpers.js
+++ b/test-frontend-helpers.js
@@ -4411,6 +4411,61 @@ console.log('\n=== app.js: routeTypeName/payloadTypeName edge cases ===');
   });
 }
 
+// ===== observation packet cache invalidation (issue #504) =====
+{
+  console.log('\n=== Issue #504: observation packets must not inherit parent cache ===');
+
+  const helperSource = fs.readFileSync('public/packet-helpers.js', 'utf8');
+  const ctx = vm.createContext({ window: {}, console, JSON, Array, Object });
+  vm.runInContext(helperSource, ctx);
+  const getParsedPath = ctx.window.getParsedPath;
+  const getParsedDecoded = ctx.window.getParsedDecoded;
+  const clearParsedCache = ctx.window.clearParsedCache;
+
+  test('clearParsedCache removes cached properties and returns the object', () => {
+    const p = { path_json: '["A"]', decoded_json: '{"t":1}' };
+    getParsedPath(p);
+    getParsedDecoded(p);
+    assert.ok(p._parsedPath !== undefined);
+    assert.ok(p._parsedDecoded !== undefined);
+    const ret = clearParsedCache(p);
+    assert.strictEqual(ret, p, 'returns same object');
+    assert.strictEqual(p._parsedPath, undefined);
+    assert.strictEqual(p._parsedDecoded, undefined);
+  });
+
+  test('observation packet gets its own path after cache invalidation', () => {
+    const parent = { path_json: '["A","B"]', decoded_json: '{"type":"GRP_TXT"}' };
+    // Prime the cache on parent
+    getParsedPath(parent);
+    getParsedDecoded(parent);
+
+    // Simulate spread + fix (like packets.js does after issue #504)
+    const obs = { ...parent, path_json: '["X","Y","Z"]', decoded_json: '{"type":"TXT_MSG"}' };
+    clearParsedCache(obs);
+
+    // getParsedPath re-parses from obs's own path_json
+    const obsPath = getParsedPath(obs);
+    assert.deepStrictEqual(obsPath, ['X', 'Y', 'Z'], 'obs gets its own path, not parent\'s');
+    const obsDecoded = getParsedDecoded(obs);
+    assert.deepStrictEqual(obsDecoded, { type: 'TXT_MSG' }, 'obs gets its own decoded, not parent\'s');
+  });
+
+  test('observation packet path differs from parent after cache invalidation', () => {
+    const parent = { path_json: '["hop1"]', decoded_json: '{"type":"REQ"}' };
+    getParsedPath(parent);
+    getParsedDecoded(parent);
+
+    const obs = { ...parent, path_json: '["hop2","hop3"]', decoded_json: '{"type":"GRP_TXT","text":"hi"}' };
+    clearParsedCache(obs);
+
+    assert.notDeepStrictEqual(getParsedPath(obs), getParsedPath(parent),
+      'observation must have different path from parent');
+    assert.notDeepStrictEqual(getParsedDecoded(obs), getParsedDecoded(parent),
+      'observation must have different decoded from parent');
+  });
+}
+
 // ===== SUMMARY =====
 Promise.allSettled(pendingTests).then(() => {
   console.log(`\n${'═'.repeat(40)}`);