Rsyslog, by default, auto-backgrounds itself after startup. That simply means that the instance that is started by the user (or script) more or less does nothing but fork a new instance detached from the current terminal session and execute it. The originally started instance exits after a short timeout. This behavior was carried over from sysklogd.
Note that auto-backgrounding is problematic (aka "makes things more complicated than the need to be") in debug sessions, lab environments and so on. So command line switch "-n" can be used to turn off auto-backgrounding. In that case, the first instance started will actually carried out the work to be done (as most would expect in the first place).
It is strongly recommended to use "-n" option for lab testing.