I know this seri is bizarre and what not. But the idea of that deer is not her never even cross her mind ? she ain't too bright for a honor student ain't she
remember, throughout this entire series everyone but koshi acts like nokotan being a deer is completely normal and nothing out of the ordinary. and nokotan has, time and time again, proven to do surreal and nonsenical things, such as using her antlers to store food and removing the top of her own head (and then sending water through it from antler to antler)...
....is it really that much of a stretch that koshi might start doubting reality, or fully expecting nokotan just suddenly appearing as a regular-ass deer to be something that might actually happen?