XP not dropping

Hi,
On our server we have a zombie pigman farm but once we switched to paper it stopped working. We no longer get xp from pigmen in the farm and just killing them. I can’t find any threads related to this so maybe someone knows?

I had the same issue. You have to disable exp merging in the spigot.yml file for the world you are playing.
In spigot.yml set

world-settings:
default:
merge-radius:
exp: -1

https://www.spigotmc.org/threads/experience-orbs-merging.359246/