It's because in Cpanel the main user and catch-all are the same by default and since we have adjusted unrouted mails to be redirected to :blackhole: so you may not receive emails by the main user of your account. You can do one of the solutions below:
1- Create another mail user with another username
2- Change the "Default Address" (In control panel of your account, section "E-mail") to the main email address of your account (Not recommended)
3- Send a request to firstname.lastname@example.org and ask to arrange it for you.