I just had this happen with a UIViewController subclass (with Xcode 9 beta 2 & Swift 4) and the solution was to tick ‘Inherit from Target’ where I set the Custom Class in IB.
If your view happens to be in a .bundle file (such as for a framework/static library) that gets copied to a different target, you’ll want to set an explicit module target for each xib/storyboard class rather than having it inherit from the target that’s hosting it.