In one of my projects, many built-in assets, such as the clear button in UITextField or UISearchBar objects, the UITableViewIndexSearch image, and the activity indicator in the status bar set by UIApplication.sharedApplication().networkActivityIndicatorVisible don't show at the correct resolution. I've noticed this on iPhone 6 and iPhone 6 Plus devices running iOS 8.4, as well as all of the simulators running iOS 8.4. I have other projects that don't show this issue.
Screenshots are here: http://imgur.com/a/7TMlE
I posted the same question on StackOverflow, and someone else had seen this issue and answered with their solution: http://stackoverflow.com/a/31952605/825033