I don't think this is a bug. main calls is_ok, which clobbers the variable k. If you change it to use lexical scoping:

is_ok(n)=

{

  my(sums=0, k=n, t=1); \\ <-- key change

  while(i!=-1 && t<k,

    if(sums>11,

      i=-1

    ,

      if(n-t^2<=t^2 && issquare(n-t^2),

        sums++

      )

    );

    t++

  );

  sums;

}

this should work as intended.