I've definitely noticed this happening. However, I think this is more "brain generates a temporary ugh field for the textbook" than being bored with the subject - when I swap to a different textbook on the same subject, I can often understand what's going on and do the exercises.

I personally interpret this as the relevant part of my brain getting low on resources. That could be experienced as boredom or simple inability. Doing something different enough from the previously explored domain does indeed work.

It seems there are also more general resources, where simply changing the topic doesn't work. If I expend my memory capacity for the day by learning one subject, changing the subject won't give me any more capacity to memorize, but I can still do deductive/mathematical reasoning or planning or learning procedural stuff just fine.