The bug
Killing a mob and then reloading the world before the death animation finishes will result in no xp being dropped.
Linked issues
Comments
Was able to replicate in 19w11b.
With XP now being dropped upon killing the mob without waiting for the death animation, is this still in the latest version or snapshot?

Resolving as Cannot Reproduce for now due to the new XP drop mechanics.
Confirmed for 1.13.1.