$ g++ --version
g++ (Homebrew GCC 9.2.0) 9.2.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

#include <bits/stdc++.h>
using namespace std;

multiset<int> init() {
    multiset<int> s;
    s.insert(2);
    s.insert(3);
    s.insert(2);
    s.insert(1);
    s.insert(2);
    return s;
}

void display(multiset<int> s) {
    for (auto v : s) {
        cout << v << " ";
    }
    cout << endl;
}

int main() {
    multiset<int> s = init();

    cout << "【ケース1】" << endl;
    cout << "変更前" << endl;
    display(s);

    // 3つとも消えてしまう
    s.erase(2);
    cout << "変更後" << endl;
    display(s);
    cout << "---" << endl;

    cout << "【ケース2】" << endl;
    cout << "変更前" << endl;
    s = init();
    display(s);

    cout << typeid(s.find(2)).name() << endl;
    s.erase(s.find(2));
    cout << "変更後" << endl;
    display(s);
    cout << "---" << endl;

    cout << "【ケース3】" << endl;
    cout << "変更前" << endl;
    s = init();
    display(s);

    auto iter = s.lower_bound(2);
    cout << typeid(iter).name() << endl;
    s.erase(iter);
    cout << "変更後" << endl;
    display(s);
}

【ケース1】
変更前
1 2 2 2 3
変更後
1 3
---
【ケース2】
変更前
1 2 2 2 3
St23_Rb_tree_const_iteratorIiE
変更後
1 2 2 3
---
【ケース3】
変更前
1 2 2 2 3
St23_Rb_tree_const_iteratorIiE
変更後
1 2 2 3