Hola, Martín, ¿quiubo? (-:
I think it's just because 'demócrata' is already invariable. Like all words for rulers ending in '-crata,' it's of Greek origin and thus is not feminine just because it happens to end in -a. The same is also true of all words for people ending in '-ista,' though I'm not sure about its etymology. Soel/la demócrata, autócrata, etc., el/la activista, partidista, feminista, etc.
There's another separate group of Greek-origin words that are simply masculine even though they end in -a.el problema, el mapa, el idioma, el programa, el clima, el planeta, el drama, el poema, el cometa, el tema ...
We could actually make a thread and collect a list.
One source I saw pointed out that the nouns in this group were neuter in Greek, but since Spanish only has two genders, they became masculine. Maybe someone who actually studied Greek could confirm that.
Back to AOC: I'm not sure her Spanish is A+ level, though it's generally good. I believe 'miembro' is simply invariable as well: el/la miembro
. So if there were a group whose members were all women, it could indeed be las miembros.
That said, usage in this area is changing a lot, with feminists wanting visibly feminine words to designate women. So where traditional usage would have been invariable,el/la piloto, el/la idiota, el/la presidente, el/la médico
there are now new feminine forms like 'la presidenta' and indeed 'la miembra,' which as far as I can tell tend to be defended strongly by some, er, assertively progressive women, but resisted on aesthetic grounds by many language traditionalists, including many who are otherwise open to progressive and feminist ideas. (That is, much like the arguments for and against the binnen-i in German.)https://www.proz.com/kudoz/spanish/other/9642...https://spanish.stackexchange.com/questions/3...