Unable to complete "Healing Power of Friendship" advancement

I'm playing on a spigot server, only plugin is "chunky", other than that it is vanilla. Mine and my friends goal for this server has been to get all the advancements, and today I have been trying to get the "Healing Power of Friendship" advancement, but no matter what I do I cannot get it to work. The Regen effect is just not being applied to me even though I have used an axolotl to help killing countless drowned. I have tried every possible method, letting the axolotl kill it, me having he last hit, killing it while the axolotl is healing, etc....

 

At this point I can only assume this is a bug...

 

Please help, we just want to do all the achievements.