NOTE: log4net isolated out from the issue, just writing the context for more diagnostic facts
I am using log4net RollingFileAppender and DebugAppener. The app can be started in console mode but also can be installed as service. When starting stanalone all DebugAppener goes to OutputDebugString confirmed via Mark Russinovich's DebugView.
When running as a service only the file log is written no output OutputDebugString.
The two likely cases are
and it should display debug messages of services as well.