I would guess, since you did not publish your new branch to the "upstream" the invoked git fetch -p
:
"[r]emove[s after fetching] any remote-tracking references that no longer exist on the remote."
- git-fetch(1) Manual Page
With other words you may push and change as much as you want (on "privatefork") with every fetch -p
from upstream you're going to remove your changes.
Besides of that: working with --mirror
and --prune
- the whole help-article on GitHub - is mentioned to create 1:1-copies/backups of repositories. If you plan to work with the repository, leave the options away and do "normal" forking.