The bug

Slimes and magma cubes split when the method net.minecraft.entity.monster.EntitySlime.setDead() (MCP name) is called. This is problematic since this method is used when an entity is marked for removal and therefore not only when the entity dies.

This can cause slimes to split multiple times.

If the splitted slimes / magma cubes are still supposed to spawn when the XP is spawned, the method net.minecraft.entity.EntityLivingBase.onDeathUpdate() could be overridden or a proper method could be added which is called by onDeathUpdate() when the 20 ticks are reached.

Fix implications

Fixing this might break the behavior described in MC-51732. It is however questionable how useful this behavior was, since it was likely rather annoying when trying to get rid of slimes.

How to reproduce

1. Set the difficulty to "Peaceful"
2. Switch to third person view (press F5)

3. /summon slime ~ ~ ~ {Health:0f,DeathTime:-40s,Size:5}
   

   For a short moment you see that the slime split before it and the smaller slimes despawn