See IRC at https://secure.phabricator.com/chatlog/channel/6/?at=110994. If you arc diff in a working copy with changes but no commits, the workflow picks shouldAmend = false because there are no commits, and does not amend the commit message after the editor prompt.

Instead, the arc-generated commit should be considered amendable.

Generally, this workflow is really confusing. This is a "bad" workflow (not committing, in the IRC case not using a local branch) but it should be accommodated more gracefully.