Minecraft 1.14.1, Minecraft 1.14.2 Pre-Release 2, Minecraft 1.14.2 Pre-Release 3, Minecraft 1.14.2 Pre-Release 4
This occurs on all versions of Minecraft: Java Edition where mending is obtainable, probably on Bedrock too.
The mending enchantment has - at least an my opinion - a pointless/unintended consequence. The way that mending seems to work is that every time a player with mending enchantment(s) gains EXP, each mending item (armor/held item) has an equal chance of being repaired. The problem is that mending items will full durability still have a chance to be repaired, effectively blocking mending items that are damaged from being repaired. This does not eat up any gained EXP, however. The full EXP just goes to the player.
If a player has no armor, and gains EXP while holding a mending-enchanted tool, all of the gained EXP goes into repairing the tool, until the tool is fully repaired.
If a player has a full set of mending armor (at full durability) and gains EXP while holding a mending-enchanted tool, there is a 4/5 chance that a piece of armor will be "repaired" rather than the tool - where the EXP will just go to the player instead, since the armor is already fully repaired.
This seems like a pointless, unintended mechanic since it effectively blocks the mending enchantment from working where it is actually needed.
Thanks for reading, I tried to read up to see if this has been posted before, and can't believe some of the posts you have to respond to.
MC-92889 Mending doesn't always consume experience if the player wears items with mending that are already fully repaired