There is array out of bound access it this line: A[i]=10*A[i]+(10*A[i-1]); when i == 0. This is undefined behaviour and probably it is the reason of different answers.

Try to change that line to something like this:

A[i]=10*A[i];
if (i > 0)
   A[i] += (10*A[i-1]);

see fixed solution. 24563693

fixed part of code:

   A[0] = (s[0]-'0') % a; // 1. --->see  %a 
    for (ll i=1;i<n;i++){  // 2. --->see i = 1
        A[i] = s[i]-'0';
        A[i] = A[i] + (10 * A[i-1]); //3. --->see removed 10*A[i].      //computes the prefixes mod a
        A[i]=A[i]%a;
    }
    ll k=1;
    B[n-1]= (s[n-1]-'0')%b; // 4. ---> see %b