In the Cambridge dictionary we find
μάκαρ μάκαιρα masc.fem.adj. | nom.masc. also μάκᾱρ (Sol.
Hippon.) , dial. μάκαρς (Alcm.) | fem.nom. also μάκαρ (E.) ,
Boeot. μάκηρα | masc.acc. μάκαρα , fem. μάκαιραν
| masc.gen. μάκαρος (also fem. E.) | fem.dat. μάκαρι (Ar.)
|| compar. μακάρτερος , superl. μακάρτατος |
1 (of gods) blessed, happy Hom. Hes. Hippon. Eleg. Lyr. A. +;
(of a haunt of divinities) favoured E. || masc.pl.sb. blessed
gods Od. Hes. Thgn. Lyr. A. E. +
And in LSJ it lists the genitive of μάκαρ as μακαρος. And logeion does not have a paradigm table for μακαρ
So if μακαρ is a masc.fem adjective then μάκαιρα cannot be the neuter form. It cannot be a dialect form otherwise they would state that it was. It cannot be an alternative form of a standard word such as color and colour since the dictionary usually writes ‘also’ in front of the word. So what is it? I’m guessing that μακαιρα is the feminine form. The dictionary usually states that too, but whenever they write masc.fem.adj that means that both the masculine and feminine are the same and that there is no neuter, correct me if I’m wrong.