The show hide functionality is handle by /libs/cq/gui/components/authoring/dialog/dropdownshowhide/clientlibs/dropdownshowhide/js/dropdownshowhide.js
If you see the code the target elements are selected based on data attribute and which is common for all the targeted elements within multifield item. Thats why on selection of one other also getting changed.
To handle this you can find the index/position of multifield item. If dropdown and targeted element both are within same multifield item then only do show hide for this set of items.