pointing to the wrong index files

Intermittently, I get messages in my archives whose thread-index and
date-index buttons unhelpfully refer to the first index pages in their
multipage archives, even though the messages themselves are found on
secondary or later indexes.

In both TOPLINKS and BOTLINKS, index buttons are constructed with:

    <a href="$TIDXFNAME$#$MSGNUM$">Thread Index</a>
and
    <a href="$IDXFNAME$#$MSGNUM$">Thread Index</a>
        
which are given as examples in the TOPLINKS resource page.  

Sometimes it works, as in most (but not all) messages on:

    http://bumppo.net/lists/realbasic-nug/1999/08/thread2.html

And sometimes it doesn't, as in most (but not all) messages on

    http://bumppo.net/lists/realbasic-nug/1999/10/thread2.html

Both of these archives were generated with the same mhonarc resource
file, which has not changed in several months.  I'm using mhonarc 2.3.3,
perl 5.004_4, FreeBSD 2.2.7.

Any ideas?  I'm planning on updating to mhonarc 2.4.4 this weekend, but
I haven't seen this glitch mentioned in recent release notes.

Thanks for your time.

  -nat