Άγιος accent in the feminin gender

Question

In noticed that the accent of an adjective doesn't move to the penultima like in ancient Greek when the adjective is put in the feminin gender. E.g. in ancient Greek παράλιος becomes in the feminine gender παραλία. Aparently, the quantity of the ultima isn't relevant anymore in modern Greek (παράπλευρος > παράπλευρη and not παραπλεύρη).

But the adjective άγιος, I noticed, is still functioning according to the 'ancient' rule, in which the quantity of the ultima is important. E.g. αγία τριάδα (not άγια).

Why is there an exception for this adjective?

Answer 1

The adjective "άγιος" is according to the ancient Greek rules because of its meaning.

But in modern Greek we can also say "άγια", besides from "αγία".

(The form "αγία" is more common)