Cannot edit file in a fork if I have the upstream repository permission when editing a file

When I am editing a file in a repository owned by an organization from Github Web UI, normally a branch on a fork of the repository will be created when I click the “Propose changes” button, which is good.
Unfortunately, if I have the write permission to the repository, the option to create a branch on a fork disappears, which is bad, because we don’t what to directly create a branch on the upstream repository.
I wonder if the Web UI could keep the option to create a branch on a fork even when I have the write permission of the upstream repository.