First enchantment on a map is always the same for a given item if you haven't unlog before.

Steps to reproduce (you can test it in creative mode and/or with cheats enabled):

• Create a singleplayer world
• Do not leave game before your first enchantment
• Enchant an item
• Create another world and do the same thing with the exact same item, you will get the same enchantment

Expected behavior: enchantments should be random.