That's normal. Discord is ditching the numbers, and going with unique usernames though still keeping the ability to have non-unique display names.

They were doing a gradual rollout, based on seniority, i think. I got a prompt and was able to pick, and it autogenerated one based on my display name at the time.

Double check your display name, to make sure it actually IS Mozdoc, and that it isn't just the display name. You can also change it in the settings freely, I made a chance almost immediately.