on the web observation edit page, if you change the map zoom at all, any save of the observation will save the zoom level of the map view, which can then be retrieved in the observation details via a map_scale field.
for inaturalist.org users, unless you’re changing the map view, you’re not going to save a zoom level value when you save.
but for users of the localized versions of iNat, your map will automatically zoom into your local iNat country, and so that’s why you generally end up with a country-level zoom level when you edit an observation in the web observation edit page, even if you didn’t change the map zoom level yourself.
so if you’re editing in one of the local versions of iNat vs the global one, the workaround to get a “default” zoom level is just to zoom all the way out as far as you can. this sets the zoom level to 0, which seems to get treated as equivalent to null.
as far as i can tell (based on the conversation in https://forum.inaturalist.org/t/maintain-map-zoom-level-when-creating-observations/8195), this behavior is the behavior of the original uploader screen, which has been retained as the web edit screen.
there are also various feature requests suggesting different ways to handle the zoom for observations, such as: https://forum.inaturalist.org/t/set-the-default-google-maps-zoom-level-for-observations/6827