Mobile menu disappearing

Hello,
Two years ago I made a website for a client with GP premium.
The client ads articles and doesn’t have access to the template or structural elements of the site.
I regularly update the site, and check that nothing is broken afterward. No change of layout has been made since it went public

All was fine until the content of the mobile menu disappeared, the menu appears but no links or texts.

It’s impossible to say when this started, it was noticed a few days ago, but it might have been going for a longer time.

Do you have any hints to what could have happened?

Thanks a lot
Didier