I have one interogation about the player’s UUID. When i’m logging in the server, my UUID is c583560c-d2ff-3102-b0b6-885d7d24a094, but my client give me 90d389a8-f979-40dc-b559-5ce44ffa9852. If you go on https://api.mojang.com/users/profiles/minecraft/djxy the id is 90d389a8f97940dcb5595ce44ffa9852(This is the UUID without the lines).
Server give me: c583560c-d2ff-3102-b0b6-885d7d24a094
Client give me: 90d389a8-f979-40dc-b559-5ce44ffa9852
Mojang api: 90d389a8f97940dcb5595ce44ffa9852
In your server.properties is the key online-mode's value set to false? You don’t want this unless you have the need to remove the built in auth for special circumstances (which is safe to assume you don’t have?).