I believe that SAP is working as designed. If you initiate the planning board using a filter with only one work order (or a list of orders) specified then this is the only order(s) SAP will consider when evaluating requirements and available capacity. If you go to the planning board from an order in the IW32 transaction then SAP will consider all orders with requirements for the work centres used by the first order. This achieves what I believe you are aiming for.