BuddyPress.org

<input type="hidden" name="field_ids" id="field_ids" value="<?php bp_the_profile_group_field_ids() ?>" />

That line is your problem. Well, more specifically, the call to bp_the_profile_group_field_ids() which creates a list of the expected field names.

Unfortunately it doesn’t have a filter so you’d have to clone and own the function into your functions.php file and manually filter out the “username” field from the string that gets generated.

Make sense?