[Bug] Achievements given on every join?

The Error/Bug:
When a player logs into a specific server on my bungee network they get false achievement notifications. The users are not actually earning those Achievements, therefore this is a quite annoying bug.
Immediately upon logging into this server I am confronted with this message(top right of screen):

I have also been confronted with this message on a different minecraft account(top right of screen):...

[Bug] Achievements given on every join?