Thanks for the screenshot. The web reports a long-standing bug in iOS where the full-page option disappears if Safari is opened from inside a Home Screen folder. It seems tied to how the app initializes. Have you moved Safari into a folder on the Home Screen? If so, take it out and try again.
If this doesn't works, it could be a device-specific glitch. Try resetting Safari settings (Settings > Safari > Clear History and Website Data).