Very often, attempting at removing a user by their ingame name will not return some good results.
When attempting /pex group user remove the player very often (If not always) will not be removed from the group on after inspecting with /pex group users. (The user will still be there by name and UUID.)

Instead, the appropiate solution is to remove the user by it's UUID. However, this is not very practical since the uuid has to be searched beforehand.

There are also issues even some ocasions where the UUID is applied as parameter instead of the name.
Therefore in these situations, removing the user from the group can only be done directly from the database.

PEX version: 1.23.4
Spigot for mc 1.12.2 (Offline Mode)

Closing issue. I just noticed I already opened one on this some time ago. I'll just update it:
#2950