"How should this be handled? Should I be writing the same thing over and over in every review?"
You could, but I doubt it would be a productive use of your time.
You could also, however, make the same basic remark and link to one of your own explanations on a similar question instead. As long as that answer is on a well-received question, similar enough and, quite a must, on the Stack Exhange network1. High-quality answers would then explain the problem based on the actual code provided while supported by the (lengthy) explanation in the link. So your last remark:
Should I really go to town on one review describing the behavior of
def, then link future reviews to that?
Is quite acceptable.
Feel free to use a format to base your answers on. Quite some regulars have experimented with such a thing in the past.
1 Not doing so could turn your answer into a 'your answer is in another castle' answer.
