On Physics SE, I just came across a surprising case which looked like a user with 1 rep posting an answer a year after the question was protected. Upon some investigation, I found that they'd posted an answer a while back, before the question, and I'm quite sure that this is what enabled them to post that answer. Here's a screenshot of a section of the question timeline, with circles for your convenience:
Delburt Phend, the author of the posts (link to physics profile) probably never had more than 5 rep at any given point of time.
I think that they should have been blocked from answering that; both answers are poorly received because they pitch the same factually incorrect idea, and the second one includes a statement "Let's hope that the moderator does not read this".
Can the system be modified such that low-rep users cannot re-answer questions after protection? I only see possible cases of misuse, like this one, and I can't think of any circumstances under which this would be appreciated. After all, they'll still be able to edit their old posts and there's almost no chance they'll be aware of the guidelines for posting two answers to the same question.
If anyone's interested, I'm attaching pictures of the two answers. They aren't important enough to be pasted properly as text, so screenshots must suffice: https://i.stack.imgur.com/OaJwm.png (new answer), https://i.stack.imgur.com/gdWqZ.png (old answer). Here's the question on Physics SE