[Risolto] language menu not showing on only one German page

Background of the issue:
I am trying to display the language switcher in the secondary menu on my WordPress site using WPML. The site I need help with is link nascosto. I have referred to the documentation at https://wpml.org/forums/topic/the-language-switcher-not-showing-all-languages-list/ but couldn't find a solution.

Symptoms:
The language switcher is not showing on only one page of my German translation - link nascosto.

Questions:
Could someone take a look and see if they can spot why the language switcher is not showing on this specific page?