Update:

Now, the last page mentioned above has changed its behaviour and does not honor NOINDEX any more. Since the page hade been moved, and for a while (many minutes, but less than half an hour) kept its old behavior after the move, I believe that this could have been due to caching. Yet I do not know enough about the actual caching in this wiki so as to be sure about it.

Thus one may now guess that the behavior of NOINEX may depend on the full page title - or anything else.

Astonishingly, only user pages appear to be listed in category of NOINDEXed pages of the wiki:
https://wiki.piratenpartei.de/wiki/index.php?title=Kategorie:Nichtindexierte_Seiten

Haven't checked carefully but what's your [[mw:Manual:$wgExemptFromUserRobotsControl]]?

NOINDEX is not working on content namespaces, you can override it with $wgExemptFromUserRobotsControl as Liangent wrote.

So WORKSFORME

Seems to be working as documented