Maybe you haven't advanced the main story far enough yet? All requirements should be shown here: https://renryuu.miraheze.org/wiki/Brad#Female_Brad
Part of the requirements is that you pick the option to support that Brad should stay as a woman, but he doesn't immediately keep in that form. It'll be part of the time skip events for this choice to take effect.

No, that's not what I meant. I meant that the option of Brad permanently staying as a woman won't happen before the time skip content. But that's not in the game yet. You can only pick the choice to try and convince Brad to embrace his female side during his events, and that is necessary for his scene with Ash later.