Debug allocator can't catch it because it's not an allocation bug. Debug allocator finds bugs by marking memory during alloc/free and inspects them upon deinit. Pointer to a memory location change is not something allocator has control over. Possible solutions: smart array list implementation (this article), move semantic analysis (Rust's borrow checker), runtime introspection (https://fil-c.org/).

It is an allocation bug, it's a use-after-free. It's only a UAF if you actually have a reallocation (something that happens in the given example), but debug allocators don't require you to annotate your code.