I‘ve had iA Writer installed on my devices for years and used it on and off. But only recently I discovered that the preview mode locks. If preview is activated then it will be applied immediately to all files loaded into the editor view until it becomes deactivated manually.
That‘s actually a nice touch. DEVONthink To Go has a very similar approach, i.e. markdown files are rendered in preview mode by default and a switch to edit mode requires a tap