By default the binder launch-url field says Fill in the fields to see a URL for sharing your Binder. If you start typing a repo name it dynamically updates to display a URL, for example entering x in the repo field results in a displayed launch url of https://mybinder.org/v2/gh/x/HEAD

However if you then delete the text in the repo field the launch URL gets stuck at https://mybinder.org/v2/gh/x/HEAD instead of returning to the default text Fill in the fields to see a URL for sharing your Binder.:

I can't reproduce this locally in the BinderHub main branch, I only see this on mybinder.org.