I believe the real reason people are so upset about the so called "gen Z stare" is less about the act itself and more about what it symbolizes.
The high-trust, cordial, welcoming society people knew from up until the early 2010s is gone and the young generations tasked themselves with delivering this cold message.
Asking low wage workers to cheer their customers is offensive, they are not paid enough to do that, all you get back is a passive-aggressive blank stare.
Being less than two meters away from a stranger is a violation of their personal space, just like the lockdowns taught them.
Interacting with strangers in general is a no-go: no more free gratitude, chitchatting and other spontaneous interactions in the name of a "cordiality" nobody cares about anymore.
The era of undeserved, unconditional wholeheartedness is over and zoomers are letting everyone know.