Ich weiß wirklich nicht wieso immer diese Aufregung um poettering. Es ist kein Bug in Systemd, weil das Verhalten so gewollt und dokumentiert ist.

https://github.com/systemd/systemd/issues/6237#issuecomment-312481714

That's exactly what happens, and what I wrote above: if the username is valid but the user doesn't exist we'll let the unit fail on start. If the username is already invalid syntax-wise we'll log about it but proceed.

https://github.com/systemd/systemd/issues/6237#issuecomment-312461325

Part of this behavior is actually documented in systemd.unit(5): "Unit files may contain additional options on top of those listed here. If systemd encounters an unknown option, it will write a warning log message but continue loading the unit."