diff --git a/changelog.d/18746.bugfix b/changelog.d/18746.bugfix
new file mode 100644
index 000000000..b80a880c0
--- /dev/null
+++ b/changelog.d/18746.bugfix
@@ -0,0 +1 @@
+Fix a bug which could corrupt auth chains making it impossible to perform state resolution.
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 2ef7e77b1..511394c66 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -1728,6 +1728,9 @@ class FederationEventHandler:
                             event,
                             auth_event_id,
                         )
+                        # Drop the event from the auth_map too, else we may incorrectly persist
+                        # events which depend on this dropped event.
+                        auth_map.pop(event.event_id, None)
                         return
                     auth.append(ae)