”Progressive Web Apps (PWAs) have been demoted from standalone apps that use the whole screen to shortcuts that open within the default browser…
This appears to solely affect users in the European Union, though your mileage may vary.
…the breakage of PWAs is deliberate. The Register understands Apple's change will cause users to lose local data in existing web apps, because web apps and Safari have different storage locations, and it will also break notifications, because there's no way to enable notifications without app installation.”