hey,
adding a group to a user via vault does add it for the world the user is currently in and not globally.
(LuckPerms-Bukkit-Legacy-2.17.46)

There's a config option for it. It's false by default since this is default Vault behaviour.

https://github.com/lucko/LuckPerms/blob/master/bukkit/src/main/resources/config.yml#L147

Oh thanks, wasn't aware of that.