Local rebase of a single-commit branch not recognized by merge request

Summary

I have created a single commit on a feature branch. In the meantime a commit has been made on the master branch, so my feature branch is "behind" by one commit. Since I'm using Gitlab EE on Gitlab.com, I have configured "fast-forward merging". When I rebase and force-push the feature branch locally, my merge commit won't update. Under "Commits" it still shows the "old" commit but clicking the branch name shows the new rebased commit.

Steps to reproduce

1. Create a feature branch with a single commit
2. Create a merge-request for the feature branch
3. Commit on the master branch
4. Rebase your feature-branch locally and force-push

Expected behavior

The merge-request should update and show the new rebased commit under "Commits".

Actual behavior

The merge-request still shows the old commit ID and tells me that "fast-forward merge is not possible".

Relevant logs and/or screenshots

Invalid merge-request

Actual history of the branch

Output of checks

This bug happens on GitLab.com