Whenever the world is (re)loaded, a tool with 0 damage (full durability) will load in the Damage tag as Damage: 0. However, when the tool is spawned, (through commands, crafting, creative menu or otherwise..) The Damage: 0 is omitted.

To visualise this, here is a comparison:

New carrot on a stick:

Same carrot on a stick after quitting to menu and reopening the world:

Notice the ... tag: {Damage: 0} ...

Clearly this is an inconsistency in the code, which makes writing mcfunctions more difficult. For example, testing if a tool has 0 damage is impossible. Testing for "Execute this function IF Damage NBT tag is absent" or "Execute this function IF Damage NBT tag is equal to 0" are 2 entirely different commands, with the same principal to figure out if the damage on a tool is 0. Except either will only work in each different case. The first will only work on newly spawned tools, but after a world reload, only the second method would work.

Here are the commands in question:

Only works when tool is initially spawned:

/execute unless data entity @s SelectedItem.tag.Damage run say hi

Only works when world is reloaded:
/execute if data entity @s {SelectedItem:{tag: {Damage:0s} } } run say hi

To reproduce:

1. Spawn a tool in a fresh world and verify it's nbt. It should not contain any "tag: {Damage: 0}"
2. Close and reopen the world
3. Verify that the nbt has changed to include "tag: {Damage: 0}"