At a guess, some caching somewhere that's locked a false availability number in for I class on the outbound flight, when the true figure is I0 and you need to book in R class. If you check the website rather than the app to see whether you get the same issue, that would point to whether it's specifically an app problem. Also, if you look on EF at availability numbers for the flight, you'd see what the true figure is and whether the app's offer of an I class seat on that flight is just wrong.