It’s pretty much always been this way.
I’ve been flying with work for +30 years and whenever someone is booking a flight for me I always remember to tell them my ‘Sunday’ name.
I never have my middle name when booking (or having booked) flights and its never been an issue, even though my passport has my full name..
Middle name isn’t a problem (mine is the name I actually use in real life rather than in bureaucracy life. And the stated first name and last name on the passport have to equate to the one on the ticket IME.
My eldest also had a problem when he recently bought a house, it seemed that every bit of official documentation had a slightly different set of names for him… He’s 3 first names and 2 second; my ex-wife remarried when they were young and she hadn’t updated any existing, so then added new and he himself has added new. Needed to officially change his name in a rush to get the mortgage 🙂