When applied the Mending enchantment, tools will not repair themselves when absorbin XP, having the player gain levels instead. Tested with all tools and materials with no armor on. Armor enchanted with Mending still works fine.
MC-156358
Mending not working on tools
Resolved
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(Unassigned)
Minecraft 1.14.3, Minecraft 1.14.4 Pre-Release 3
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Make sure you are holding the items when you receive the exp, either in your main hand or offhand.