Sorry for the confusion here! We've started integrating more features from our website into the CLI, including each branch's overall status, which we calculate to give you an idea of what is needed to merge it.
"Not on default branch" is a legacy status that we intended to remove a while ago, and somehow stuck around — we've landed a fix to our server to remove that as one of the possible results of this calculation, and as soon as your CLI next syncs you'll get the correct one.