Basically, With changing capitalization being possible now for mojang accounts, you could change your username to the exact same name and capitalization that it was before, enabling the 30 day cooldown. I tried this on my account "ggggggggggg" and it's only visible in the API but it shows as me changing my username from "ggggggggggg" to "ggggggggggg" as you can see here https://api.mojang.com/user/profile/8a13bb92c5ac4bcbba99c9b782631eca/names. Its really easy for someone to accidentally change their username to the exact same thing as their last username and it makes you unable to rechange your username.
WEB-5141
you can change your username to the exact same capitalization it was before
Resolved
Works As Intended
0
This was a side effect of allowing you to change capitalization, but released it intentionally. We don't see why you shouldn't be able to change to your name to the one you currently have. It's unnecessary perhaps, but also a method to block your account from making any other name changes, in case you are afraid of someone stealing your name (although having a strong password, not sharing the account with anyone and having 2FA should be your primary goal to keep your profile name safe!)