diff --git a/changelog.d/19927.bugfix b/changelog.d/19927.bugfix
new file mode 100644
index 00000000000..d69e4febbe1
--- /dev/null
+++ b/changelog.d/19927.bugfix
@@ -0,0 +1 @@
+Fix a regression where application services that opted into ephemeral events using the legacy `de.sorunome.msc2409.push_ephemeral` registration flag stopped receiving ephemeral events (including to-device messages used for encryption). Introduced in v1.156.0.
diff --git a/synapse/handlers/appservice.py b/synapse/handlers/appservice.py
index 36b2f63e41c..68b8aa71f1c 100644
--- a/synapse/handlers/appservice.py
+++ b/synapse/handlers/appservice.py
@@ -313,7 +313,10 @@ def notify_interested_services_ephemeral(
                     StreamKeyType.PRESENCE,
                     StreamKeyType.TO_DEVICE,
                 )
-                and service.supports_ephemeral
+                # Honour both the stable `receive_ephemeral` registration flag and the
+                # legacy `de.sorunome.msc2409.push_ephemeral` one, matching the
+                # transaction body built in `ApplicationServiceApi.push_bulk`.
+                and (service.supports_ephemeral or service.supports_unstable_ephemeral)
             )
             or (
                 stream_key == StreamKeyType.DEVICE_LIST