BuddyPress.org

This is not a “problem” per se.

It’s a matter of customizing the language file:
https://codex.buddypress.org/how-to-guides/customizing-labels-messages-and-urls/

Most users will input a subject… so I don’t see this being a big issue. You could possibly add javascript validation so the subject has to be filled out before the form is submitted.

Also, just wondering how you changed the username? This will help with others who encounter this problem later.