Why does Museo Sans fall back to Arial when our app is deployed?

The fonts are served only to pages on aleris.ai, aleris.se, aleris.no or aleris.dk and their subdomains, and to localhost, so an app on a platform's default hostname gets Arial with no error; serve it from an Aleris domain.

The browser asks the font host whether the page may use the font, and the host answers yes only for Aleris domains. The fallback is silent because Arial is next in the font stack, which is why the page looks almost right.