"$User restored source branch XXX" when pushing branch created from Issue

Problem to solve

We have an issue coming up today for the first time regarding the connection between MRs and branches.

We are using gitlab.com. Our workflow is and has always been:

1. Create Issue

2. Create MR from Issue

3. git fetch

4. git checkout 123-fix-this

5. git push

6. Expect to see changes and commits in the corresponding MR, but it is empty and shows no merge button, it shows:

   USER restored source branch 123-fix-this

What's going on here? Under Repository -> Branches, I can see the branch with its changes, but the MR shows this weird message.

Other links/references

Might be related to gitlab-foss#16551 (closed)