diff --git a/changelog.d/19936.misc b/changelog.d/19936.misc
new file mode 100644
index 0000000000..5c1f7eef52
--- /dev/null
+++ b/changelog.d/19936.misc
@@ -0,0 +1 @@
+Fix Complement test flake when restarting Synapse workers (cross-test pollution caused by nginx upstreams being temporarily unavailable).
diff --git a/docker/configure_workers_and_start.py b/docker/configure_workers_and_start.py
index 26c8556eff..38f8649b44 100755
--- a/docker/configure_workers_and_start.py
+++ b/docker/configure_workers_and_start.py
@@ -1070,14 +1070,21 @@ def generate_worker_files(
     # Determine the load-balancing upstreams to configure
     nginx_upstream_config = ""
     for upstream_worker_base_name, upstream_worker_ports in nginx_upstreams.items():
+        # We use `max_fails=0` to prevent nginx from marking an upstream as unavailable
+        # after it fails to contact the Synapse worker. Otherwise, if nginx sees a
+        # Synapse worker as unavailable once, it will be marked as unavailable for 10
+        # seconds (`fail_timeout` default).
+        #
+        # This is necessary because we use `COMPLEMENT_ENABLE_DIRTY_RUNS` (re-uses
+        # deployments/homeservers) and we don't want any cross-test pollution from
+        # stopping/starting homeservers.
         body = ""
         if using_unix_sockets:
             for port in upstream_worker_ports:
-                body += f"    server unix:/run/worker.{port};\n"
-
+                body += f"    server unix:/run/worker.{port}    max_fails=0;\n"
         else:
             for port in upstream_worker_ports:
-                body += f"    server localhost:{port};\n"
+                body += f"    server localhost:{port}           max_fails=0;\n"
 
         # Add to the list of configured upstreams
         nginx_upstream_config += NGINX_UPSTREAM_CONFIG_BLOCK.format(