That jib is highly loaded with a lot of leech tension. That leech tension is adding to the halyard tension (and stretch), allowing the luff to go slack. With the jib unloaded for furling, the luff should be taut. Some race boats use a latch at the head of the jib, basically attaching it to the mast. When the leech is loaded, there's no added halyard tension (it goes into the mast), and the luff stays taut.