I don't think any single was majorly affected by oricon's decision to not count oversea sales.
Plus, I feel like you (directing to BG) just don't understand the situation.
I honestly think it's because hardcore fans don't like the song. They want a fun song and Yattaruchan, Ee ka?! and Eighteen Emotion delivered that. They don't mind a mature song, but they don't want a whole single of the group that is suppose to be smiling all the time sing deep songs.
Box set sales are about half of their sales (while for Momusu, it's the majority of their sales). Mystery Night's tuesday sales was at 16,238, which it's not bad. From the looks of it, almost all the competition on day one looks like they were all close. So maybe they made near 16k time time around. Who knows.
Like I said a few posts above, they are a group that heavily depends on promotions. They are currently working their butts trying to do all of current release events. People need to buy the single on-spot in order to get in the signing events, mini live events, or handshake events. Like JonCC said, it's best to wait until the weekly sales instead since S/mileage the weekly sales usually don't say much.