If I don't intentionally save a document in TextEdit, I expect it to be left as is. That is, I expect a changed document to prompt "Do you want to save?" so that I can choose Delete. This means you can accidentally save an unwanted version of a file.
I fixed this on my Mac when the "feature" initially became available, then something recently (April 2018) changed the setting back. See my answer below for the setting needed.