It's been a long time since this issue happens.

Whenever a server issued a ban, with an invalid name, that never joined MCbans server before, the banned player name will become Stephenblong.
Upon a new player connecting to the server, the server will sometimes (turns out to be if API server cannot response fast enough), will think that player is stephenblong, thus kicking the player.
Sometimes, admins may make a typo, sometimes, the banned player changed his name before an admin bans him. These are two ways to run into this issue. The banning message is shown as normal, making it difficult for admins to realize they had done wrong.

The current fix is to constantly unban stephenblong in case someone issued a typoed ban. It is not good.

I am not able to reproduce this issue again, since I am running out of alt accounts.
Only those are new to mcbans server will be affected.

I can confirm that this is possible, but I do not have alternate accounts to test this with.

It does not look like MCBans is well-prepared for UUIDs.

Fetching player information (such as bans) on the MCBans servers still uses a username, not a UUID.
See PlayerListener line number 77.

Banning players through /ban will send the username, not the UUID.
See BaseCommand at line 97, does not set the target's UUID (I don't know why it's commented out, maybe it caused issues??).

The above could be a possible explanation as to why this happens.

If you have any more alternate accounts, try logging onto a test server with MCBans, change your username and wait for a period of time (probably about 10-15 mins) and then ban the account. I am genuinely interested to see the result.