From a diachronic point of view, I think that the French pair un/une has grammaticalized while it specialized.
Masculine > indefinite article and (attributive) numeral yon;
Feminine > (independent) numeral youn
Due to this process, there is great resemblance between the two, but not identity.
Dejean (1980: 198) writes: "Puisqu'il n'existe pas de segment final /n/ dans l'article indéfini mais qu'il s'en trouve dans le numéral, ce dernier rime avec /mun/". ("Since there is no final segment /n/ in the indefinite article, but there is one in the numeral, the latter one rhymes with /mun/”.)
This statement of a native investigator confirms my intuition and my auditive experience.
Source: Fattier 1998: vol. 2,p. 842
Source: Fattier 1998: vol. 2, p. 843
Source: Fattier 1998: 842