Mahara manual search result links are broken

The search result links turn out like this example:

https://manual.mahara.org/en/20.04/administration/institutionsundefined?highlight=site%20skins

Note there is the word 'undefined' in the URL

I'm guessing there is a variable that needs to be set so that the link should turn out as

https://manual.mahara.org/en/20.04/administration/institutions.html?highlight=site%20skins

Looking at https://www.sphinx-doc.org/en/master/usage/configuration.html there is an option called 'html_link_suffix' that we might need to set to '.html'

But the links in other places work without it - only the search results links don't work so I'm not sure if this is the right fix for the problem