Rather, in this case, I felt fully awake and ready to keep reading. And I did keep reading. It just… didn't work. I'd have trouble following simple proofs. I'd fail at parsing sentences that were quite clear after resting.

Yeah, it sounds like you were just cognitively exhausted. Try taking an extended break (a few hours) and doing something else. It's also possible that you can learn that material that many hours a day.
I've noticed similar kind of tiredness when I switch job and every day there is a LOT of stuff to learn, often with zero context. Sleeping more helps me to deal with that kind of stress.