Support for overflow: hidden on the <body> element is quite limited in iOS and Android. To that end, when you scroll past the top or bottom of a modal in either of those devices' browsers, the <body> content will begin to scroll.
Also, note that if you're using inputs in your modal or navbar, iOS has a rendering bug that doesn't update the position of fixed elements when the virtual keyboard is triggered. A few workarounds for this include transforming your elements to position: absolute or invoking a timer on focus to try to correct the positioning manually. This is not handled by Bootstrap, so it is up to you to decide which solution is best for your application.
Also, note that if you're using a fixed navbar or using inputs within a modal, iOS has a rendering bug that doesn't update the position of fixed elements when the virtual keyboard is triggered. A few workarounds for this include transforming your elements to position: absolute or invoking a timer on focus to try to correct the positioning manually. This is not handled by Bootstrap, so it is up to you to decide which solution is best for your application.
The .dropdown-backdrop element isn't used on iOS in the nav because of the complexity of z-indexing. Thus, to close dropdowns in navbars, you must directly click the dropdown element (or any other element which will fire a click event in iOS).