The bug

The value of the XpTotal tag of a player which is used as value for scoreboard objectives of the criteria type xp and the query result of the /xp command when giving experience (not levels) does not account for level changes.
It is basically the number of experience the player has collected. It only resets to 0 when the player would have a negative experience level.

Affected events:

• Enchanting
• Using an anvil
• Adding or removing levels using /xp

How to reproduce

1. Create a scoreboard objective tracking the xp of a player

   /scoreboard objectives add xp xp
   /scoreboard objectives setdisplay sidebar xp
   

2. Use for example the /xp command

   /xp add @s 10 levels
   

   The score does not update

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The tag XpTotal (field net.minecraft.entity.player.EntityPlayer.experienceTotal) is only increased by the method net.minecraft.entity.player.EntityPlayer.addExperience(int) and additionally never decreased.