Comment 4 for bug 204918

I'll bet this is the Text Replacement plugin. To fix the problem, go to Tools->Plugins and either disable the Text Replace plugin by unchecking it in the list (my preference, because I don't like machines deciding what I should have typed); or click on the plugin in the list and then the "Configure plugin" button, where you can delete any rewrites that don't suit you.

I'm putting this comment here in the hope it will be of help to other users who, like me, spend some considerable time trying to figure out where the rewrites were coming from and how to stop them.