looks like you are using int index for a string indexed AA.
for(int i=0;i<10;i++)
tcb_seq[11:0] = {tcb_seq[10:0],tcb_field[i]};
so its giving you default value.
You can try foreach (tcb_field[key]), but SV LRM does not gurantee order in which a foreach elements will be visited. it just requires all of them to be visited once in a foreach.
I would say use first(), next() to traverse the AA .