"The app does not know that you are taking a screenshot, rather iOS knows you are taking a screenshot (as it must) and is excluding an element on display that has been designated by the developer as sensitive information. This is the same technology that prevents you from accidentally screenshotting your password manager"

And that is reasonable, but it is also a surface where an app touches the OS, which should be a permission boundary that I can control. Allowing the option to opt-out of screenshot blocking with a proper double-confirm warning and biometric auth is also reasonable.