diff --git a/changelog.d/6714.bugfix b/changelog.d/6714.bugfix
new file mode 100644
index 0000000000000000000000000000000000000000..410516694fc6ddb3cd072220c5266a0f67c84282
--- /dev/null
+++ b/changelog.d/6714.bugfix
@@ -0,0 +1 @@
+Fix a bug causing Synapse to not always purge quiet rooms with a low `max_lifetime` in their message retention policies when running the automated purge jobs.
diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index 00a6afc963d49dc3df7ef963abfaadf3b0387db1..3ee6a091c528af441b24311412a7f99518f8695d 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -156,7 +156,7 @@ class PaginationHandler(object):
 
             stream_ordering = yield self.store.find_first_stream_ordering_after_ts(ts)
 
-            r = yield self.store.get_room_event_after_stream_ordering(
+            r = yield self.store.get_room_event_before_stream_ordering(
                 room_id, stream_ordering,
             )
             if not r:
diff --git a/synapse/rest/admin/__init__.py b/synapse/rest/admin/__init__.py
index a10b4a9b7263fa9ec30e96f5fa4e0c88a0d15f35..2932fe2123d8ab2b69cd9d6ce38524eec1c77b8f 100644
--- a/synapse/rest/admin/__init__.py
+++ b/synapse/rest/admin/__init__.py
@@ -107,7 +107,7 @@ class PurgeHistoryRestServlet(RestServlet):
 
             stream_ordering = await self.store.find_first_stream_ordering_after_ts(ts)
 
-            r = await self.store.get_room_event_after_stream_ordering(
+            r = await self.store.get_room_event_before_stream_ordering(
                 room_id, stream_ordering
             )
             if not r:
diff --git a/synapse/storage/data_stores/main/stream.py b/synapse/storage/data_stores/main/stream.py
index 140da8dad68692f57f9dac1f84e6eca9f61f146e..056b25b13a3fecd2b70b577d20b7e4b03e77816f 100644
--- a/synapse/storage/data_stores/main/stream.py
+++ b/synapse/storage/data_stores/main/stream.py
@@ -525,8 +525,8 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
 
         return rows, token
 
-    def get_room_event_after_stream_ordering(self, room_id, stream_ordering):
-        """Gets details of the first event in a room at or after a stream ordering
+    def get_room_event_before_stream_ordering(self, room_id, stream_ordering):
+        """Gets details of the first event in a room at or before a stream ordering
 
         Args:
             room_id (str):
@@ -541,15 +541,15 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             sql = (
                 "SELECT stream_ordering, topological_ordering, event_id"
                 " FROM events"
-                " WHERE room_id = ? AND stream_ordering >= ?"
+                " WHERE room_id = ? AND stream_ordering <= ?"
                 " AND NOT outlier"
-                " ORDER BY stream_ordering"
+                " ORDER BY stream_ordering DESC"
                 " LIMIT 1"
             )
             txn.execute(sql, (room_id, stream_ordering))
             return txn.fetchone()
 
-        return self.db.runInteraction("get_room_event_after_stream_ordering", _f)
+        return self.db.runInteraction("get_room_event_before_stream_ordering", _f)
 
     @defer.inlineCallbacks
     def get_room_events_max_id(self, room_id=None):