LibreLogin Offers three UUID creators, each has its benefits and drawbacks. These only apply to new players.

RANDOM

This one is pretty straightforward, it generates UUID randomly. Please note, that if you didn't migrate your database, players will lose their data.

CRACKED

This one generates UUID the same way the offline mode server does.

Drawback

Let's imagine a situation where player pepazdepavole joins the game for the first time. Their UUID will be f586c1d8-2314-3df5-bfc4-9ff22f132273. This player enables autologin and changes their nickname to kyngs. However, even though their name changes, their UUID still stays the same.

Now a new player who also chooses nick pepazdepavole joins the game for the first time. There is no one with their name in the database, so logically the plugin attempts to create a new user. So the plugin generates the UUID f586c1d8-2314-3df5-bfc4-9ff22f132273. However, player kyngs already has the UUID, so pepazdepavole will be kicked and asked to join with the name kyngs.

TL;DR

If a player changes their nickname, their original nickname stays occupied.

Mojang

This one firstly checks whether a user with that name exists in the database, if yes, uses their premium UUID, otherwise fallbacks to CRACKED.

The drawbacks are same as in CRACKED