Hi,
You have misunderstood the concept of strategy 50... it is used for planning all sub assembly materials (in your case HALB and HALB1) even before the sales order comes for your header material ( FERT).. so you need to make Individual/coll. as 2 (Collective requirements only) for both HALB and HALB1 in material masters in MRP4 view.. then only it will work... make these settings and re run MRP for FERT with planning mode 3 then check..
Thanks
Kumar