I discovered I had the same problem but not hose length related. My problem was the check valve in the water pump head was leaking. For a temporary fix I installed a check valve on the output side of the pump. Problem solve. I will bet it will backfill the tank with the longer hose but as you said it doesn't, it is a mystery to me.
To me, it seems the dealer should be responsible for the problem until he can prove it is a faulty pump and the pump manufacturer is responsible but the dealers job to get reimbursed not yours.