Probably a bug in C++

#	User	Rating
1	tourist	4009
2	jiangly	3831
3	Radewoosh	3646
4	jqdai0815	3620
4	Benq	3620
6	orzdevinwang	3529
7	ecnerwala	3446
8	Um_nik	3396
9	gamegame	3386
10	ksun48	3373

#	User	Contrib.
1	cry	164
1	maomao90	164
3	Um_nik	163
4	atcoder_official	160
5	-is-this-fft-	158
6	awoo	157
7	adamant	156
8	TheScrasse	154
8	nor	154
10	Dominater069	153

Code1

#include <iostream>
#include <vector>
#include <functional>

using namespace std;

int main(){
    function <bool()> func=[](){
      vector <bool> a(1,true);
      return a[0];
    };
    
    for(int i=0;i<10;i++)
        cout<<func()<<'\n';

    return 0;
}

Look at this code. It seems, the expected output is always 1. However, in

Codeforces
- G++20
- G++17
- G++14
- VC++2017
in my local Windows G++ 8.1.0
in my local Debian G++ 10.2.1

the output is always 0. In Codeforces::Clang++17 & Clang++20 the output is

Clang++

Code2

#include <iostream>
#include <vector>
#include <functional>

using namespace std;

int main(){
    function <bool()> func=[](){
      vector <bool> a(1,true);
      bool t=a[0];
      return t;
    };
    
    for(int i=0;i<10;i++)
        cout<<func()<<'\n';

    return 0;
}

Now look at this code. Here in all previously mensioned compilers (including Clang++) the output is always 1.

How found