Are you looking at exactly the same location in both instances?
I don’t know what the form_to_mappress shortcode is doing, but it must be creating a MapPress shortcode at some point. Is it possible it’s not passing through the zoom level setting to the MapPress shortcode?
If you could provide a URL of a working map and a URL of a non-working map at the same location, I can at least tell you if the non-working version received the zoom setting.