If the user's email ends with something that an existing user's email is, then the server is created on the other client's account.
For example, a new client with the email "example@gmail.com" would get their service created on the existing user who has the email "myexample@gmail.com".
The issue lies with the filter present here: https://github.com/pterodactyl/whmcs/blob/master/pterodactyl/pterodactyl.php#L310
If the user's email ends with something that an existing user's email is, then the server is created on the other client's account.
For example, a new client with the email "example@gmail.com" would get their service created on the existing user who has the email "myexample@gmail.com".