When working in Visual Studio, when I diff a file from the Git Changes pane it has always shown the diff within Visual Studio using the VS integrated diff tool.

When working from the command line, I had Beyond Compare set to be the default diff tool by following

This is what you want to get back to with Visual Studio (not VSC), correct?

Try unsetting all of these settings and see how VS reacts. Does it go back to the default internal diff or not?

git config --global --unset diff.tool
git config --global --unset merge.tool 

Otherwise, you might just need to reset your settings in Visual Studio.