That's the case for every dropped equipment with an extended third slot (the ones with a plus sign on its third slot). But if it doesn't have that plus sign on the third slot, there is no problem with adding an option to the item at all. Unfortunately, I don't think a dropped equipment with a legendary craft will ever drop with a non-extended third slot. At least, I haven't had such an experience.
I also asked in-game why I can't add a third option to an item like the one in the picture and someone answered that I have to upgrade the item to +20 first before I can add a third option. I don't know if it's true though.