C++ multiset iterators. My code crashes due to iterator error. What am I missing?

Question

vector<int> smallestRange(vector<vector<int>>& nums) {
        using vit = decltype(nums[0].begin());
        using pit = pair<vit, vit>;
        multiset<pair<int, pit>> window;
        for(vector<int> vec: nums) window.insert({vec.front(), {vec.begin(), vec.end()}});
        int lo =  window.begin()->first;
        int hi = window.rbegin()->first;
        
        while(true) {
            auto [vit, end] = window.begin()->second;
            ++vit;
            if(vit == end) break;
            window.erase(window.begin());
            window.insert({(*vit), {vit, end}});
            int i = window.begin()->first;
            int j = window.rbegin()->first;
            int dist = j - i;
            if(dist >= 0 && dist < hi - lo) {
                lo = i;
                hi = j;
            }
        }
        return {lo, hi};
    }

What am I missing about Iterators? Are multiset iterators changed or invalidated on erase? The code finds the smallest range given a list of vectors sorted in nondecreasing order. I maintain a multiset to track elements from each list ordered. Remove the smallest, adding its next element while calculating the range.