in the gitlab the -dev branch is always the default branch, so when you want to do merge request on -fixes you have to position yourself on another branch.
git checkout 5727-create-a-changelog-in-markdown-format
git reset --hard 1.2-fixes
git push origin +1.2-fixes:5727-create-a-changelog-in-markdown-format
The branch should show no change in the gitlab interface
** You maybe have to refresh twice **