It’s not complicated, you’re trying to book an I class fare but I is not available on the second leg so it’s defaulting to Y instead. If you didn’t spot it during booking you can call and cancel within 24 hours, wait times are currently very short and you’ll be done in under 5 mins - certainly less time than writing multiple posts here.

As suggested above, the alternative is to ask for a more expensive fare overall (e.g. D class), which would give you Business on both legs.