Sorry to hear of that but also not the first I have heard of it. The initial fare sounded very good but not shockingly so. Ive had roundtrip France to Brasil in F not long ago for under EUR3000, and roundtrip Brasil to France for well under 2000, so what you had sounded like a fair fare (pardon the play on words) for Mexico.

I’ve even had on airline web sites tickets I’ve priced all up in business but when the receipt came through one of the long haul segments was economy. I caught it quickly and canceled, rebooking with another flight at the same fare in business for all segments. Not sure exactly what happened but it is too often that the site will show you business, price it out in business, and then throw back a leg in economy. Buyer beware.