Not sure if bug or intended behaviour…
In the specific instance of a category with auto-closing topics:
It would appear that if the topic starter is staff, then the auto-close message at the end of the topic is attributable to the topic starter, rather than @system.
Additionally (and probably more intentionally) if another staff member tweaks the autoclose time, the auto-close message is attributed to them, regardless of topic starter. This I can understand (audit/prevent abuse etc.)
I presume the behaviour described in the first part of this post is a side effect of that described in the second part.