Shouldn't it display the current operating system version? For instance, if I install the app on an iOS 26 beta device, this API should return iOS 26 rather than iOS 19, regardless of which Xcode version I used to build my app.

So it means this is intentional? We use this information to find out which iOS version the customer is using. In case they want to share iOS information to help debug any issue, showing iOS 19 to the customer instead of iOS 26 might confuse them.