in both cases it scales up to the center, however when scaling down it indeed stays in the top-left corner in letterbox if the layout size is too short and unbounded scrolling is set to its default value, didn't know this was different in the case of fullscreen off though.
due to the fact it is not consistent I'd report it (with the capx) just in case, but you would not want that centering to happen in zoom out without unbounded scrolling I think.