To reproduce the error you need to enable topics for a list, create a topic and send a post that matches that topic.
You can do the enable/create by putting these two lines
topics_enabled = 1 topics = [('Match_all', '.', 'Topic matches anything for testing', False)]
in a file and running Mailman's bin/config_list like
bin/config_list -i path/to/file/with/lines list_name
Then, a post to that list should be delivered normally and contain a header 'X-Topics: Match_all', but the bug will cause the post to be shunted.