Why do I get different behavior of mask_array assignment?

Question

#include <valarray>  
#include<iostream> 

typedef std::valarray<int>    valarray_t;
typedef std::valarray<bool>   maskarray_t;

int main(void) {

  valarray_t::value_type   ibuf[10] = {0,1,2,3,4,5,6,7,8,9};
  valarray_t               vi1(ibuf, 10);
  valarray_t               vi2(ibuf, 10);
  

  // Create a valarray of bools for a mask.
  maskarray_t::value_type  mbuf[10] = {0,1,0,1,1,0,1,0,1,1};
  maskarray_t              mask(mbuf,10);
  
  maskarray_t::value_type  mbuf2[10] = {0,1,1,1,1,1,1,0,0,0};
  maskarray_t              mask2(mbuf2,10);
  
  // test 1:
   vi1[mask] = static_cast<valarray_t> (vi1[mask2]);
   std::cout << "vi1[mask] = vi1[mask2]\n\n" << vi1[0] <<'\t'<<vi1[1]<<'\t' <<vi1[2]<<'\t'<<vi1[3]<<'\t'<<vi1[4]<<'\t'<<vi1[5]<<'\t'<<vi1[6]<<'\t'<<vi1[7]<<'\t'<<vi1[8]<<'\t'<<vi1[9]<<'\t'<< std::endl;

    //test2: 
    std::cout<<"**********************test result=result2"<<std::endl;
    std::mask_array<int>    result1= vi2[mask];
    std::mask_array<int>    result2 = vi2[mask2];

    std::valarray<int>      temp1(result1);
    
    std::cout<<"temp1[0]="<<temp1[0]<<" temp1[1]="<<temp1[1]<<" temp1[2]="<<temp1[2]<<" temp1[3]="<<temp1[3]<<std::endl;
     
     result1=result2;

    std::valarray<int>      temp2(result1);
    std::cout<<"temp2[0]="<<temp2[0]<<" temp2[1]="<<temp2[1]<<" temp2[2]="<<temp2[2]<<" temp2[3]="<<temp2[3]<<std::endl;
     
     
     result1=result2;
     std::cout << "result1= result2\n" << vi2[0] <<'\t'<<vi2[1]<<'\t' <<vi2[2]<<'\t'<<vi2[3]<<'\t'<<vi2[4]<<'\t'<<vi2[5]<<'\t'<<vi2[6]<<'\t'<<vi2[7]<<'\t'<<vi2[8]<<'\t'<<vi2[9]<<'\t'<< std::endl;
     
  return 0;
}

Testing the code above, I got

vi1[mask] = vi1[mask2]

0   1   2   2   3   5   4   7   5   6   

**********************test result=result2

temp1[0]=1  temp1[1]=3 temp1[2]=4 temp1[3]=6

temp2[0]=1  temp2[1]=3 temp2[2]=4 temp2[3]=6

result1= result2

0   1   2   3   4   5   6   7   8   9

For my understanding, both of the results are wrong.

orig:     0 1 2 3 4 5 6 7 8 9 
mask:       ^   ^ ^   ^   ^ ^
mask2:      ^ ^ ^ ^ ^ ^ 
vi1[mask]=vi1[mask2] or result1=result2
          0 1 2 2 2 5 2 7 5 2   

orig[3]=orig[2] //orig[2] is 2, so orig[3] is 2

orig[4]=orig[3] //orig[3] is set to 2 from last step, so orig[4] is set to 2.

...

So my questions are:

1. why do I get different outputs from test1 and test2? Should test1 and test2 give the same result?
2. Are both outputs wrong? if not, why?
3. For test2, the original array doesn't seem to be updated. Why? Is it a bug?

I used a pretty old compiler and llvm c++ library, try to figure out if it is a bug or I understand/use mask_array wrong.

Please kindly share your thoughts. Thanks for any comments.