Can't merge a branch

Summary

I want to restore the current Master branch to a previous version but the UI won't let me do it.

Steps to reproduce

Select previous commit tag. Create new branch. Issue Merge request to merge branch to Master.

Example Project

See https://gitlab.com/BackStopThemes/ThemeCoreV2/-/merge_requests/7

What is the current bug behavior?

I Select previous commit tag. Create new branch. Issue Merge request to merge branch to Master.

If I look at the branches it shows my new branch as merged with a flag that says it could not be merged automatically and the Merge Request still exists and Master file list has not changed.

What is the expected correct behavior?

It should be merged, the branch should be gone (I checked the remove branch option on the merge request) and Master should have files set to the equivalent of the tagged commit.

If you look at the following 3 screenshots you will see that it says the merge could not be done, but the 2nd screenshot says the branch has been merged, but the third screenshot shows the merge request still exists and can only be edited...there is no "Complete Merge" button in sight.

And how can the branch say "merged" when my merge-request had the "delete branch on merge checkbox checked".

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

N/A

Results of GitLab application Check

N/A

Possible fixes

None suggested