Based on your description, it sounds like a bad interview question. When otherwise capable candidates can't answer your trivia, maybe it isn't a useful signal? More than anything, it just sounds like you're selecting for people who practiced leetcode vs those who didn't.
It's not whether you can solve it right away, it's how you approach the problem solving process to reach a viable solution. If "try to prove that this code is actually correct" is part of that process, that's the kind of "trivia" that can really be widely applicable even in a working environment, in a way that random leetcode algos often aren't.
I vote for making "how would you think about proving this correct?" a question in all algo-focused interviews.
To be precise, Zoz said
>about 2/3rds of highly credentialed applicants
Credentialed ≠ capable.