The thing is the following: The exact same form component is used in multiple places, including the "new post" and the "edit post" context. In most of these contexts, it makes sense, to assume that there is only something in the input, after the user has pressed a key. An exception is the edit context, where there is content before the user has interacted in any way. Because of that assumption, the form will update the preview whenever a key is pressed.

Hence everything works, when you press the "Ctrl" key to zoom in/out. However, any other key should work, too, including using the arrow keys to move the cursor.

This makes some sense for edits, too, as you probably only need the preview once you make changes ('cause you just saw the preview). But this is still suboptimal behavior and will be changed in the near future.