By default, there appears to be no fashion choice (feminine/masculine) when selecting the base layers category.
While I know feminine CAS assets have been made androgynous in the past, these are always tagged with a fashion choice, and the base layers do filter when masculine/feminine is ticked, and the feminine are not properly shaded for masc frames/vice versa. So I know this is not intentional.
Also, if there are multiple sims in the household and you select a fashion choice for base layers on one sim, that fashion choice filter (only for the base layers subcategory) carries over to any sims you select; ignoring their selected fashion preference and filter for the accessories category.
Video demonstration of the last issue for clarity (imgur)
(I do have mods enabled but none that are supposed to affect CAS. I do have some CAS CC.)