Diff view does not display file content after staging

When I add a new .sj file
and when it is still unstaged
then it shows the content in the diff view of Source Tree
But when I stage it
the diff view does not display the content anymore
And when I commit it
the diff still does not display it
The encoding of the file is utf-16. I know that I can set SourceTree to use utf-16, but then other files have issues. I am just wondering why it displays the content while it is still unstaged.