Rebase from Merge Request UI does not work

Summary

We are experiencing a bug on our Merge Requests, we are not able to use the rebase button on any of our Merge Request. We do have this error message: Fast-forward merge is not possible. Rebase the source branch onto the target branch or merge target branch into source branch to allow this merge request to be merged. but the Rebase button is not disabled (it does nothing when clicked).

Steps to reproduce

• Create a new project in gitlab
• Set the option Fast-forward merge (in General > Merge request settings)
• git clone the repo
• Create a commit on master echo 'test_master' > master_file && git add master_file && git commit -m 'test_master' && git push origin master
• Create a branch and add a commit to it: git checkout -b 'branch-1' && echo 'test_branch' > branch_file && git add branch_file && git commit -m 'test_branch' && git push -u origin branch-1
• Create a new commit on master: git checkout master && echo 'test_master_2' > master_file_2 && git add master_file_2 && git commit -m 'test_master2' && git push origin master
• In gitlab create a merge request for the branch branch-1
• See that the rebase button is active and does nothing and the error message Fast-forward merge is not possible. Rebase the source branch onto the target branch or merge target branch into source branch to allow this merge request to be merged. is still there

Example Project

You can see this behavior on this project (2 merge requests) https://gitlab.com/Rowern/test-merge-request

What is the current bug behavior?

The rebase button is active but does nothing.

What is the expected correct behavior?

The button rebase should rebase target branch into the merge request branch and push it to the repo.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com