well ... it’s one of life’s ... mysteryIn these cases, mystery, cousin, senior, relative and site would all be plural nouns in standard English.
I’m not ... sure because one of my cousin is from my mother’s side and the other is from my father’s side ... of the family
my coach is actually one of our senior
and one of our relative pick us up from the KL airport
the Sharm El Sheikh is one of ... the tourist ... site where ... it’s a bit similar like Ha- er Hawaii
It seems that speakers feel that the referent is singular, so the noun should have no plural suffix. I guess this is logical in a way.
I haven't seen this reported for other New Varieties of English; but I suspect it does occur elsewhere.