@NGOtaku maybe Boku no Kokoro no Yabai Yatsu, where they look like high schoolers but are actually in middle school, so the inverse of this? I occasionally get that and this confused in my head too . . .
In chapter 30, we saw that Shiraishi had the same problem of his legs falling asleep after seating in similar style. May be his blood is going elsewhere when Kubo is around.