As it turns out, Lil Puss did hold all 10 math notebooks. The tenth one just spawned in a random location rather than Classroom 0011, like in Brainiac mode. It was on a table it wasn't on before.
The hidden notebook H0race was missing might've been in the middle of the courtyard.
Anyway look at this
I swear, the item combination function is broken. And yes, I did try remapping and rebinding the keys.
Also,why would this happen for no reason?
(Like, literally, I was just stacking mice and it happened out of nowhere.)