I think I got it. The number of lines in the $conf['changelog'] for the particular namespace was lower than "num".
If I may say so, the main cause of troubles here, is the fact that the config option "recent" has been abused in a way that is likely to cause confusion. Even if https://www.dokuwiki.org/config:recent
does point out a few places where it is used, this documentation does not reflect the complexity of the actual code. Moreover, the short text "Number of entries per page in the recent changes" in ?do=admin&page=config is definitely misleading, in this regard.
I tried to mitigate this with https://www.dokuwiki.org/syndication?rev=1366552346&do=diff
but I suggest to reconsider $conf['recent'] and its documentation globally.
Anyhow, the fact that $conf['recent'] (and $conf['recent_days']) is (ab)used at various places cripples e.g. "feed.php?mode=recent&num=##" especially in conjunction with "&ns=NAMESPACE" as one might _have_ to use a low $conf['recent'] for performance reasons and then ends up with empty feeds for less frequently changed namespaces.
BTW, this also shows that the full HTML feed of "feed.php" is by no means a full replacement for the "blog" component of the "feed" plugin [ https://www.dokuwiki.org/plugin:feed