Mail text appears in Inter (icon font) in Apple Mail.app, but not in Safari (iCloud)

Dear community,

some mail I receive in Apple Mail (the application on a desktop Mac running Sequoia) displays its text in a font named Inter (a pure icons font). Copying the content of that email into a TextEdit file and converting it to another font makes it legible. Opening the same email in icloud.com/mail/, the mail appears as it should, not in inter. No setting in Mail.app preferences gives me an idea why that may be. Any idea?

Thanks!