The mark text changes feature re-enables itself automatically. After disabling the feature under Tools > Options > Text Editor > Display, it remains in effect until the file, project or program is closed and reopened. Ticking and unticking the checkbox can sometimes deactivate the feature once more, but usually a toggling of another feature from the same category (e.g. visualize whitespace) is required for the former to register properly.

I have not encountered this issue in QtCreator 3.2.2