Because the Constitution wasn’t ratified until 1788.
Are there earlier grandfather clauses somewhere?
Jefferson wrote one into the citizenship law for Virginia in 1779.
https://press-pubs.uchicago.edu/founders/documents/a4_2_1s4.html
I wasn’t using the phrase as a legal term of art, but casually.
The word retroactive would have been more precise to the period.
I do tend to use them interchangeably - but I’m not a lawyer, nor a Judge, so I won’t come down too hard on myself.