Why is the iterator to the last element of the vector holding a corrupted value? (refer code)

The below code throws heap-use-after-free error.

#include <iostream>

int main()
{
    unordered_map<int, vector<int>::iterator> mp;
    vector<int> num;

    auto insert = [&](int n)
    {
        if (mp.count(n) != 0)
            return false;

        num.push_back(n);
        mp[n] = prev(num.end());
        return true;    
    };

    insert(1);
    insert(2);
    insert(3);

    for (auto n : num)
        cout << n << endl;

    cout << endl;

    for (auto m : mp)
    {
        cout << m.first << " -> ";
        cout << distance(num.begin(), m.second) << endl; 
        // cout << *m.second << endl;
    }

    return 0;
}

The output of the above code is

1
2
3

3 -> 2
1 -> -16
2 -> -7

It obviously crashes if I try to access *m.second. Why are the iterator values seemingly corrupted here?

I have tried replacing vector with list and the above code works fine. I wonder if this has something to do with how vector expands on "push_back" maybe. I have tried other ways to get the iterator to the last element of the vector with indifferent results.