Cannot change to ANY 3 letter usernames

Because I wanted to choose a three letter username for my account, recently created a python script that checks for every three letter username (It works). However, after generating ALOT of untaken three letter usernames, I was not able to change my username to every single username on that list.

I have checked if they are unusable or taken, and I even checked if there is some sort of due date to the names.

Here is an example (there are tons of more examples on the attached txt file)

k7t

https://namemc.com/search?q=k7t   :  namemc checker shows that it is possible to use

https://iaero.me/mcchecker : iaero.me also shows that the username is not taken

GET https://api.mojang.com/users/profiles/minecraft/k7t : Using mojang's REST api, we can check that it returns 204 (no content) which means that the username is untaken

 

But when I try to set it as my username, it does not work.

 

I will attach a txt file consisting of a part of the usernames I generated and my screenshots.

 

If you need the whole list to check through, please contact me.