Well, this is interesting! The search field didn’t appear because I have my computer language set as Portuguese. As soon as I choose English as the main option in the system preferences, the search field appears. Seems like, since the default name for the “help” menu in Portuguese is “Ajuda”, the OS doesn’t recognise the menu item “Help” as the default one if it’s not the system language, but it does if it is. Conversely, if I set Dorico and the system language to Portuguese, it also works.