I have been at business lunches where people were so offensive, I would not work with them. As an example, I was at a business lunch one day and one of the people I was eating with reached over and took food off of my plate and ate it. I could not believe it!
At a business meal, I intentionally slow down and eat slowly. Generally speaking, eating fast does not inspire confidence or make the right impression.
Dining etiquette is in the details. Ordering the right kind of food as not to have a manners nightmare, like not slurping spaghetti, or that over-sauced burger and insisting on using your hands, or not using your napkin.
Manners as my earlier post states is about respect.
I still marvel that I married a man who licked gravy off his knife.
That could certainly cause severe issues, though the fact that you were having business lunches with them shows that their etiquette did not make them undesirable in terms of hiring.
I guess we have to make of that what we will. We these all millenials or were some of them from older generations as well?