Question: TextEdit Bug
TextEdit has a bug where, if you set the default format to plain text then cmd+n for a new document, it opens new documents in .rtf. This doesn't always happen. It might happen when after opening an rtf file and converting it to plain text by cmd+shift+t you then use the hotkey to make a new document. Regardless, it happens and it's wrong.
Mac mini (Late 2012), macOS High Sierra (10.13)