I’ve been thinking of Yashahime: Inuyasha and his niece Towa are weakened by the New Moon which was also when they were born while his other niece Setsuna who had her memories taken is not weakened by the New Moon.
Furthermore the New Moon of the Modern Era doesn’t weaken Towa so what is going on? Is there some entity generating energy detectable by Half-Yokai(though not Quarter-Yokai) during the New Moon(and different energy at other times of the Month) in the Feudal Era that needs to be killed?
Whatever it is is well informed about times of Month to say the least especially since it consistently starts using it’s New Moon Energy when approaching the New Moon(only turning it on full blast when the New Moon actually appears) suggesting that it knows the exact moment the New Moon is going to happen.
Half-Yokai with memories weakening is probably a subconscious reaction of fear so Setsuna regaining her memories will gain memories of being afraid of the strange presence during the New Moon which will clash with her memories of not being afraid and thus her subconscious will know that whatever is giving off the presence is not looking for her and that she doesn’t need to hide her powers.
Of course that might also mean she might suspect there is a Yokai Aura she never realized was even there since it was always present(and shifting for each time of the Month) and being a Yokai Hunter start looking for it making sure Moroha and Towa follows her with Towa’s powers completely diminishing as they get close to the source before the Sun has even set revealing the New Moon.