I though that the sometimes it opens in B and sometimes it creates a new pane was due to using the wrong shortcuts or being confused by the shortcut change between edit mode and preview mode.
If this is not the case, that is, you are in edit mode and you always do the same actions and the result is random, then we should look into it.
If this is the case, please specify exactly what you do that results in a random behavior (attaching a gif would be even nicer).