The bug
Some tools which are crafted with iron ingots and which have durability cannot be smelted to iron nuggets.
Affected items
Last updated for 1.14 prerelease 2
shears
flint_and_steel
shield
crossbow
anvil (depending on how you define durability; uses block IDs instead of NBT)
Can Confirm for 1.12.2. Just tried to melt down shears, and it didn't work.