Add changelog

changelog.d/19142.misc

@@ -0,0 +1 @@
+Respect logcontext in `DeferredEvent`.

synapse/util/async_helpers.py

@@ -1029,7 +1029,8 @@ class DeferredEvent:
 
     def set(self) -> None:
         if not self._deferred.called:
-            self._deferred.callback(None)
+            with PreserveLoggingContext():
+                self._deferred.callback(None)
 
     def clear(self) -> None:
         if self._deferred.called: