Side by side diff does not show modifications

Step to reproduce:

• Create a sample git repository
• Create a file modified.txt:

This line has been added
This line will be modified
This line will be deleted

• commit to master&push to origin.

git add modified.txt
git commit -am "add lines to modify"
git push

• Create a branch: git checkout -b modifications1
• Modify modified.txt:

This line has been added
This line has been modified

• Commit, push and create merge request

git add modified.txt 
git commit -am "modify the file"
git push origin modifications1

See an example merge request here:miscreant/side-by-side-diff-test!1 (diffs)

Expected Output: The added/modified line should be shown in the side-by-side diff

Actual Output: The modified line is not shown at all.

Inline Diff:

Side-by-side Diff:

GitLab version: