Hey,
a player just came to us with that he bought Minecraft but couldnt use /premium on our server because apparently his name is not premium, I went to check with the API link and put in his username and then also UUID and only UUID worked.
https://api.ashcon.app/mojang/v2/user/rzucz3k
https://api.ashcon.app/mojang/v2/user/cc8c72d4a7ee4e8193026838f789c92e