Looting enchantment does not work

I am playing on a 1.13 vanilla multiplayer server. ( ip: 188.165.48.42:25665 )

At this moment, here are the plugins installed on the server : Core Protect/Creative Gates/EditableSign/Massive Core/Multiverse Core/Private Chest/ServerList Plus/World Edit/World Guard

The problem is that Looting enchantment seems to be inefficient : severals players have been reporting this issue. 

We tested swords with different enchantment levels (None / Looting I / Looting II / Looting III)on 10 creepers each time.

Results are 10 gunpowder on average (specifically : none -> 11 gunpowders; Looting III -> gunpowders)

(And this is not intended at all by admin or owner of the server!)

 

We hope you will be able to help us out, and anyway thank you in advance

(Excuse me for any errors in English)