Diff later not using configured Diff-Viewer

What steps will reproduce the problem?

1. Set alternative Diff Viewer:
   
2. Select two files to "Diff later":
   (1) First File:
   
   (2) Second File:
   

What is the expected output? What do you see instead?
It should diff the two files with the alternative Diff Viewer but will diff them with TortoiseGitMerge instead.

What version of TortoiseGit and Git are you using? On what operating system?
2.13.0.1, Git should not be of any importance.