The behaviour is correct as the plugin cannot send any data. This makes the service check Apache Status stale and generates the WARNING in the agent query check Check_MK.
If you need to know if Apache is still running use a process check, configured with the ruleset “Process discovery” or use the ruleset “Check HTTP service” to query the open port.
The apache_status plugin will not send any error message if it cannot find a running Apache instance. This would generate false positives on hosts where the plugin is installed but no Apache.