Per https://en.cppreference.com/w/cpp/container/unordered_map/insert, the insert method "inserts element(s) into the container, if the container doesn't already contain an element with an equivalent key."

The call to insert in the following section won't actually change the contents of the unordered_map.

if (map.find(arr[i]) != map.end()) {
    auto freq = map.find(arr[i])->second;
    freq++;
    map.insert(pair<int, int> (arr[i], freq)); // <<-- here
}

Option 1: Make freq a reference

if (map.find(arr[i]) != map.end()) {
    auto& freq = map.find(arr[i])->second;
    freq++;
}

Option 2: Simplify the algorithm,

unordered_map<int, int> collectMap(const vector<int>& arr) {
     unordered_map<int, int> map;
     for (int val : arr) {
         ++map[val];
     }
     return map;
}