Ok fine, then lupa non grata (singular) or lupae non gratae (plural) "Lupae non grata" just sounds wrong and agramarical, in the same sense as "people is happy

Ok fine, then lupa non grata (singular) or lupae non gratae (plural) "Lupae non grata" just sounds wrong and agramarical, in the same sense as "people is happy" would to an native English speaker. Yes it's just a little sounds, just like the small difference between "are" and "is"