I had some problem like this on Iphone and Ipad using Sharepoint 2013 Modal bootstrap keep z-index problems with backdrop.
I just use this on JS :
$('#myModal').on('shown.bs.modal', function() {
//To relate the z-index make sure backdrop and modal are siblings
$(this).before($('.modal-backdrop'));
//Now set z-index of modal greater than backdrop
$(this).css("z-index", parseInt($('.modal-backdrop').css('z-index')) + 1);
});